designing a rifle and would like some input

[justin22885]

i have this other idea worth throwing out there... since the top rifle will likely be a side mag with the gas system under the barrel..or if it goes to a bullpup it will use a carrier like a steyre AUG, again with the bolt being on the top side of the carrier.. it frees up the top surface of the bolt that i could easily cut a cam surface for an AR-10 bolt and it compliments this other idea ive been having for the gas system

for the gas system, ive been thinking of mounting a short gas tube to the front of the trunnion.. have this short extension contain a short-length, short strike gas piston and spring, it would realistically only need to extend maybe two inches out of the front of the trunnion... this way you can use an AR-10 or AR-15 gas block and extend the gas tube back to the front of the trunnion where the piston is at.. this allows you to have different length gas systems and only require you to cut a simple AR-15 style gas tube to length

downside to using AR-10 bolts and barrels is they dont make a .223 option and i dont think an AR-15 barrel would even fit into an AR-10 receiver to justify designing a carrier that could use either bolt.. so youd need to get an AR-10 barrel extension, chamber it for .223, then weld up the bolt face and tip of the extractor claw, machine these for the .223 bolt face and re-heat treat.. so it might still be easier in the end to use the savage 10 bolt face, a pipe for a bolt body for it to mount to and just have to lathe out the required barrel contours from a blank

but its an idea worth considering as an alternative to using a savage bolt head which is available in bolt faces for every caliber.. and im still not opposed to using savage barrels either.. just need to realize youd have to lathe an untapered section out for the gas block to sit on (or ream a taper into the gas block) and couldnt use a receiver extension for the forearm because then you couldnt get the wrench in to tighten the barrel extension, and using savage barrels would require a steel trunnion

 

[justin22885]

well.. ive made some significant progress here.. ive designed much of the upper.. the receiver i seem to be settling with is a piece of square tube steel but a strip down the center of the top is removed.. allowing the remaining lips to add as rails, like on an AK receiver.. then theres a dust cover thats slid on over top.. but slid on like an FAL dust cover.. in fact, removal of the dust cover isnt even necessary for a field strip.. so it can even be welded on as a solid receiver.. but i like the option of being able to remove it for easy access to the internals

so far ive just modeled a basic aR10 magwell, just a simple one thats not even final, im going to make it much longer, and its a bolt-on magwell.. could easily fab an FN FAL magwell that would fit in place of this one.. bolt one off, bolt the other one on.. and ill be designing an AR-15 magwell for it later.. the tab where the bolts go into on the back side of the AR-15 magwell will be extended to make up for the smaller length.. magwell is black because i set this one to be made of injection moulded polymer (add about .25lbs for aluminum)

i still need to work on the sheet metal box for the lower, itll use an AR-15 buffer tube and stock, though the buffer tube wont function as such, itll just be to hold the stock.. also going with a triangular bolt and something more along the lines of the M14 gas system.. you can see the op-rod attached to the bolt carrier.. there will be an adjustable gas plug on the front of the gas block, and a short-stroke piston in the gas tube

you can see my rifle starting to take shape.. when its finished i estimate, and im shooting for roughly 8lbs, which is what an FN SCAR-H weighs

when this is finish, i predict it will have the weight of a SCAR-H, interchangeable magwells, easy change barrel, so in all simple caliber changes, and likely lower felt recoil than a SCAR-H due to the stock also being in line with the bore but also using a longer stroker lower pressure gas system

at this stage it is currently 4.75lbs and all its missing is the short-stroke piston, the gas plug, i need to make a couple small tweaks to the bolt itself and the handguard and then the ENTIRE upper assembly is finished

___

required skills to assemble the rifle from a parts kit.. less receiver would be simply cutting a strip in the receiver to act as the rails, cutting the magwell, and drilling out some holes for the trunnion rivets.. due to the bolt locking in the barrel the trunnion is to be made of aluminum to save weight, and it will use an off the shelf AR-15 FCG

this is it so far

 

[barnbwt]

Cool, it's looking a lot like a G36 so far . If I may be so bold as to ask; why the AR buffer tube constraint? The tube itself is the product of a design compromise and arguably the weakest point of the AR platform. I can sort of understand a desire for something you can easily interface with, but a 'blank' back end capable of being adapted to use AR, AK, FAL, or any other stock would be far more flexible, would probably take up less room (your bolt carrier body could get much smaller), and would be capable of accepting folding stocks (I like them over collapsibles, so do some other folks)

Have you figured out how big of a bolt thrust your bolt lugs can carry, yet? That will put a top-end on the cartridge power you're going for, which will put a limit on how big of mags/cartridges the magazine well would need to be adaptable to, and how much bolt throw you need to accommodate the longest. The recoil attendant your top-level cartridge will also drive how beefy the rest of the gun needs to be.

As far as receiver construction, there's some cool threads elsewhere about a project called the AK42 (or was it MG47?). Basically, it was an AK action flipped upside down in a new receiver and converted to belt-fed with an MG42 topcover. The receiver was two stacked square tubes with their mating face milled away and then welded together to form a BREN-like railed carrier tube.

TCB

 

[justin22885]

never heard of it, but i did find an image when i searched MG47 so i think thats it

as for the stock what i was planning to do was to put a hinge mechanism on it that would hold an AR-15 buffer tube for the stock selection.. what do you think about the ACE modular trunnion AK builders use to replace their original trunnion to allow the plethora of ace modular add-ons for their stocks?.. i could easily design something compatible

 

[justin22885]

at any rate.. heres a photo of the rifle mostly done.. still need to finish up and tweak a few things.. not sure i 100% love the way the magwell hooks into where the lower receiver pivots for field stripping and havent 100% arrives to a decision on how the barrel will attach, im honestly leaning towards an interrupted thread and something to secure it in place like a lever or a detent

anyway, below is the photo.. and i did some math on this to calculate just how much force my bolt could handle, but i didnt just measure how much force my bolt could handle.. i made sure to measure the maximum allowable bolt thrust of the section of the bolt that actually contacts the locking surfaces in the barrel extension.. after crunching all the numbers this is what i came up with for my current bolt design

maximum PSI of the metal surfaces (using 4140 steel hardened to a rockwell hardness of 49 (its low, i know) was 42,728PSI.. with the surface area of the cross section of my bolt lugs where they contact the locking surfaces in the barrel, it gives me a maximum bolt thrust of 17,305 pounds.. this would make my bolt capable of 338-lapua

for anyone wishing to run the math themself.. the "arc length" though in this case its a straight edge where it contacts the barrel extension is 0.361" and the axial length of these surfaces front to back is .375" and there are three lugs

im thinking of reducing the diameter of my bolt and my locking lugs.. the capability of .338 lapua is overkill for a rifle i cant possibly conceive ever being chambered in anything with greater bolt thrust than a .308.. not even if i made an extended version of this rifle to express my affection for the 8mm mauser round

so yeah, im at a point in this design where the mechanics are all there and now i need to start getting serious about things like metallurgy as well as shaving off some unnecessary weight.. and prior to these weight reductions, with all components already added to the rifle im at exactly 8lbs for the total weight of the rifle, after weight reductions we'll be looking at a 7.5-7.75lb rifle with a 16" barrel.. and that is an AR-10 magwell with an AR-15 mag release.. so aftermarket "bullet buttons" can be installed for cali people

oh, and im looking for a gas system that will be multi-caliber, multi barrel length friendly.. so what i intend to do is shorten the oprod and drill a pocket in the face of the oprod to act like a gas key on an AR-15.. at which point id like to be able to use .750" AR-15 gas blocks and gas tube with that gas tube going inside the pocket in the front of the shortened oprod.. what this will do is allow me to have different length gas systems that can easily be adjusted simply by trimming the gas tube that directs gas from the gas block to the shortened op rod.. this also keeps more mass rearward for a better balanced rifle

 

[barnbwt]

Lookin' good so far. I have one of those ACE adaptors I plan on using for my AR70 one of these days (a cool design to draw inspiration from for your swappable magwells, btw ). The ACE things are well made, but they made the fatal error of using bead-blasted anodized aluminum for the bearing surfaces. Aluminum sucks as a moving bearing surface for a number of reasons, so long story short, the ACE folders are almost comically hard to open/close. Doesn't help that they use automotive valve springs to tension the latch . Still, there's probably nothing better that's as universal to flat-backed receivers out there

Have you changed the bolt head from when you posted pics earlier? The lug diameter is large enough that it looks like it could probably handle gun-level loads (usually when I see the concept of polygonal bolts pitched, the flats are tangent to the shaft, leaving very little meat behind). I would also suggest you make the flats into concave grooves as close to tangent with the bolt shaft as possible, with a standard ball-mill size radius. This will increase the space efficiency of your lugs, and will be just as easy to mill as a flat face (use ball mill instead of end mill)

Were your stress calculations for ultimate, yield, or fatigue conditions? You don't want to operate at the yield point, or your headspace will go all wobbly in a handful of shots (your conservative hardness estimate helps you here, but a S-N curve for fatigue in steels is a better guide for finding where you wish to operate)

Secondly, was the failure you were checking for shear or contact load at the lugs? Bending and compression are obviously not the critical numbers for such a short 'beam' as a bolt lug. Shear probably won't be either, unless your bolt lugs are really short front to back. The contact load was always what ate me alive when I'd design bolt heads; that's why 1911's have so many barrel lugs and why a skinny interrupted-thread bolt needs so many threads. Spreading the load out enough that it wouldn't peen stuff seems to be the driving factor for things like bolt diameter and receiver(extension) strength.

TCB

 

[justin22885]

i tested the shear of the locking lugs.. but not the entire locking lugs.. but the tip along the line that contacts with the locking surfaces within the barrel.. and yes its the same bolt you see in the carrier above.. so youre suggestion of removing some material on the flat edges of the bolt based on common endmill sizes is a pretty good idea so ill see how that works out

as for the stock, im really leaning towards having the rear of the lower set up for the buffer tube, with a side folding buffer tube adapter.. so you can have a fixed or folding stock.. id like to use a buffer tube with a door in the front of it so while the stock is folded you can open up the storage inside the buffer tube which will likely contain things like a cleaning kit

now heres an issue im having right now.. the rifle ejects at an angle 30 degrees above horizontal.. this places the ejection towards the bottom edge of the interior rails, where you see the ejection port now it removes a chunk of the internal receiver rails, you can see the groove in the bolt carrier that rides on these rails.. i dont like this because i dont want dirt, dust, and debris to get trapped into that groove and then get pulled inside the rifle when firing and i cant move the rails because this upper is a piece of 1.25" square tube steel with the top edge removed to make the rails and a dust cover over this

so anyway, im probably going to change the carrier around a bit to save weight (currently the carrier + oprod weighs 1.2lbs vs the 1lb for the AK carrier+piston) and i think id like to get the weight down to about 1lb.. also i think im going to design an ejector thatll eject at a lower angle

___

on the order of the receiver the reason i went with the receiver you see is because its incredibly simple to fabricate and doesnt require welding, and the ability to remove the dust cover allows easy access to work on the internals while the dust cover doesnt have to be removed for field stripping and disassembly it allows a full length top rail...

but other ways i can design this receiver is a piece of rectangular tube steel or a piece of long D tube steel which in either case would require using a couple steel angles welded to the inside for the rails

of those receiver options, what do you think would be best?

 

[justin22885]

yeah, im not going to change the bolt.. the bolt face as it stands allows a maximum rim diameter of 13.6mm with enough lug to handle 17,000 pounds.. im looking for this to be a bolt i can use for other rifles as well including this other side project im working on of a straight pull bolt action.. if i can use the same bolt and the same barrels it'll allow said straight pull bolt action to be a .300 winchester magnum.. my idea is to make interchangeable bolt heads similar to what the savage 10 rifles use but in a more flexible 3-lug design.. the bolt body itself may be little more than a pipe

due to the design of my bolt carrier, its not final, not even close.. that was just a rough sketch to get something put together to start getting a better idea of the mechanics of this, i still have a lot of changes to make on that and hopefully make it less top heavy because i want more of the recoiling mass to be closer to the bore and in line with the bore and stock.. low recoil is a goal.. i want to design a 308 rifle thats going to be very accurate, very reliable, and comfortable for even smaller size people to shoot.. so a lot of thought has to go into finding a good muzzle brake thatll reduce even more recoil but still be comfortable enough to shoot without ear protection

it looks like im going to begin working on a prototype soon.. the decision im facing now is do i keep this a square tube steel receiver with half a piece of round tube steel for a dust cover (which can even be welded on since its removal is not necessary for disassembling the rifle).. or do i go back to my original idea of a piece of rectangular tube steel or long D tube steel and plug weld rails to the inside of it

 

[barnbwt]

"I'm looking for this to be a bolt i can use for other rifles as well including this other side project im working on of a straight pull bolt action"

Hmmm...rotating bolt...camming flutes on bolt head shank...straight pull bolt action... ever mess with a Steyr M95, before? The bolt heads are almost exactly what you are describing, albeit two big Mauser lugs rather than three.

As far as bolt heads, be sure, too, to check out the Thompson Center Dimension series (I'd forgotten about until just now). Ugly-assed rifle, brilliant concept. It's a three (square) lug bolt head that locks into a barrel-extension like an AR. The receiver is basically a threaded tube with a keying feature to keep the bolt head aligned as it enters the barrel (a gas-op wouldn't even need that). Just like the AR, the barrels/bolts can be swapped out readily, and are supposed to be available in a ton of sizes (probably not as many as Savage since it hasn't been out as long, but there's quite a few already). I think buying a Dimension bolt/barrel to build a semi-auto off of would be a pretty cool concept (they all have the same critical exterior dimensions so as to fit in the stocks, etc.) and might actually make more sense than trying to get a Savage to work, since you'd still have to figure out the barrel extension locking lug half of the equation. Heck, if that bolt handle is removable, you could attach your cam pin right there

No good pictures of the bolt face, for some reason, but you get the idea;

"i tested the shear of the locking lugs.. but not the entire locking lugs.. but the tip along the line that contacts with the locking surfaces within the barrel.."
For very small locking lugs like the AR's which have a high aspect ratio (length to cross section area) failure modes like bending and shear are more critical, and contact failure by peening is less critical (since there is so much more effective area to load up). When you go to a smaller number of larger lugs, the opposite occurs, and bending/shear stop being the driving factor. At the very least, as a sanity check, compare the contacting surface area of your bolt head to that for other 308-class rifles with similar-size bolt heads (a Remington 700's bolt area would be good to compare with, for example, or maybe a FAL's. An AR10 might be a useful data point, but the lugs are so different that other factors may come into play, namely that the bolts aren't contact-load critical. An FAL for sure is, since both sides of the lug are massive blocks of metal abutted)

"so a lot of thought has to go into finding a good muzzle brake thatll reduce even more recoil but still be comfortable enough to shoot without ear protection"
Do a Form 1 and make it integrally suppressed (heck, make the suppressor part of the action --an insanely large volume muzzle booster )

I would suggest you use round tubing if at all possible. 4130 DOM round tubing is available in a vast array of sizes and thickness, and has very good internal/external dimensions and smoothness. No boring/turning would likely be necessary. Square tube, like I'm using for my Skorparev design, is extremely limited by comparison, and is very expensive (~40$/ft) to get in small quantitites. I was only able to find one supplier who'd answer my calls that sold the 7/8" tubing that will telescope in my 1"x.063" wall receiver tube as the bolt carrier/piston. Also, it's apparent that some amount of warpage variance occurs in these tubes, so tight fits are a very bad idea (which is why I have a round piston head/gas tube inside the square receiver tube).

TCB

 

[justin22885]

ive thought about having the cam groove on the small half inch shank of the bolt head so the cam surface was between the bolt head and the bolt body.. but instead the way i have it now is that the bolt head fits inside a piece of 5/8" steel tube, the cam groove will likely be cut in this groove (imagine an FG42 or a K31 bolt.. which as it stands my straight-pull bolt action using this same bolt will most likely end up being pretty similar to the K31 bolt action rifle but with something more like the savage 10 barrels or the similar remington 700 barrels i believe i seen made by bergara

but enough about that.. about the design i have now, ive lowered the ejection port a great deal, it now ejects at a 22.50" angle which allows the cartridges to eject underneath the rails so the top of the ejection port is right under my top rail.. also, the carrier isnt so bulbous and top heavy looking, ive moved the majority of the weight more towards the center, much closer to the bore of the rifle and saved a total of .1lb

its likely this wont even be the final bolt carrier because if im going with something more like the FG42/K31 bolt, then i may use a similar style bolt carrier but at this point i fear that may make the bolt carrier TOO light weight.. too low of a bolt carrier mass and reliability will go down (AKs are reliable not because of their piston design but because of the higher mass carrier)

___

the gas system i wanted before which would have used an oprod on the carrier that was bored out in the top to act as a gas key for an AR-15 gas block and tube, cut to length apparently will not work.. the shorted i could make the oprod without it popping out of the trunnion with the carrier fully rearward is about 6 inches.. minimum barrel length i could have is MAYBE 8 inches which is plenty short enough anyway but kind of renders my entire gas system idea useless

furthermore, i wanted to reduce the weight of my bolt carrier to around 1lb as it would still be plenty reliable with enough mass but reduce some of the extra weight.. so what im thinking of doing right now is removing the oprod entirely and going with a short-stroke system.. the way ive considered setting up this short stroke system was to build the piston into the front trunnion by adding a tubular extension in front of it for the piston to sit in.. it would have .25-.5" of travel before gasses are vented in front of the trunnion and then i could use the AR-15 gas block and gas tube to bring gasses rearward to the trunnion mounted short stroke piston.. benefit of this is i can use any AR-15 gas block (since my barrel is set up for a .750" gas block anyway), and it allows me to bring the center of gravity of the rifle more rearward.. though im still not opposed to using something more like an FAL gas system on this rifle

current weight 7.5lbs

heres what my gas system would have looked like as originally intended

 

[barnbwt]

"AKs are reliable not because of their piston design but because of the higher mass carrier"
That makes them a bit more reliable at stripping rounds off the mag and not getting stuck, at least (but obviously mag design is even more critical to function/reliability). It also makes recoil harsher as the gun cycles, though . That's why belt feds are (all?) long-stroke actions with big, heavy pistons and carriers that can smash a round through/from those links without stopping. On that note, one gas-system idea you may want to visit is making the gas tube short enough that the piston head actually exits before a full stroke; the actual driving portion of the piston throw is of course shorter than what's needed to unlock the bolt, and if your carrier-on-rails has a rigid piston extending off it, all you need is a chamfered gas tube opening to guide it back in (and a shroud to keep hands away ). This is how the big bruisers work, or at least the ZB37 and UK59 that I'm acquainted with (both have BREN-like carriers and gas systems)

UK59 bolt carrier is at the top, there; the piston is only like 2" long

The BREN has a more 'conventional' piston length, but it is fully out of the gas tube and inside a guide tube within the first 1" or so of its travel, if memory serves

A K31-like bolt sleeve is a really cool idea; you could conceivably have an aluminum bolt carrier since no sliding contact of significance will take place against the carrier (rather the sleeve and bolt). Very high potential for weight and recoil savings, there.

TCB

 

[justin22885]

well there are a few factors that make the AK more reliable than other rifles.. magazine design, the fact the magazine is curved, the fact it mounts to the same platform as the barrel (magazines mounted to a lower receiver can feed rounds at off angles if the lower isnt properly fitted), no carrier jumping from the upper to the lower like on an AR-15.. again, if the upper and lower arent fitted properly this will cause problems, but the thing that contributes the least to its reliability is its gas system.. in fact, its little different than any other short stroke system.. and my comment about the carrier weight allowing for better reliabiltiy was aimed more at the fact that the extra mass carrying extra momentum is better able to overcome obstructions, debris and friction than a low-mass carrier

you know, i really do like that gas system design i sketched out above.. i just dont think its all that practical, i mean.. you can easily adjust the stroke-length (length of time the piston is under the impulse of the expanding gasses) of the gas system simply by drilling the pocket in the oprod deeper or shallower.. as the oprod pulls away itll separate the oprod from the gas tube and vent the gasses

now im starting to think my idea with the shortened short-stroke piston which is mounted to an extension of the trunnion may not be the best idea for this application.. im considering using an interrupted thread with a detent or a lever for removing and installing the barrels.. it would be very difficult to push a barrel on and twist it and expect the gas tube to fit inside the cavity where the gas piston is at

so im thinking i may have to go with something like the FAL, or M14 gas system, or still go with the idea i originally had since the barrel is installed while the bolt carrier is removed or at least locked back.. i dont know, i see the merits of being able to use widely available ready mad gas blocks but i dunno.. this seems to be the biggest block i have right now in determining what i need to do moving forward

___

question though.. if i used two guide rods that went through the entire bolt carrier and fit into the back of the front trunnion.. would these be sifficient enough to be used as rails?

 

[justin22885]

heres a concept of the one gas system i tried to describe earlier.. its a short-stroke piston built into the trunnion.. theres a block in the back side of it that holds the return spring, and a cap that threads over the end of that really short gas tube that sticks out the front to hold the piston in.. and you use an AR-15 gas block and gas tube to bring the gasses back from the gas block and to this piston.. so this is a photo of my front trunnion which i have made translucent so you can see the piston.. foreseeable downside to this is it would bring a lot of heat into the trunnion.. a lot more heat in the receiver.. may not be the best idea

 

[barnbwt]

I think the bigger issue would be you'd be tapping gas right at the neck of the cartridge, potentially. For rounds like 308, it'd likely still be on fire, and not the most gentle substance for your piston/spring parts to be playing with. I think the real issue with heat in the trunnion would be de-tempering of your piston spring (probably not an issue on a semi-auto used responsibly, but it has similar drawbacks to barrel-centric springs on olde-tyme machine guns like the DP28)

What if you eliminated the spring entirely, and just used a loose tappet? If the burning gas is not an issue (who knows, with stainless steels it may not be) a loose tappet would never have a spring to de-temper, and would be light enough that the returning carrier could push it back forward without losing much momentum. The piston would be fully forward in battery, so no annoying rattle when moving the rifle.

For lower-gas volume rounds like pistol rounds, a captive piston in the trunnion would be a great way to shrink everything down and keep it modular.

There's no reason two guide rods out of the front trunnion wouldn't support the carrier sufficiently, provided they are thick enough. As you know, lots of designs use dual rods to guide bolts/carriers. If you're suggesting they would only be supported at the front trunnion, that'd probably still work since the carrier is under greatest load at that end, where it has the most support. You'd likely need to beef up the rods and trunnion seat a bit to keep everything rigid, though.

One other thing that might be a good idea is to make your return spring for the bolt in line with the piston. You are already impacting the bolt offset from its center of mass, trying to spin it against the rails, so the last thing you want is extra off-axis force from a low-set recoil spring. The VZ58 is a great example of what I'm talking about (the bolt carrier has a deep hole drilled for the return spring to collapse into, and the far side of the hole is the face that is struck by the short stroke piston). Keeping the return spring raise up makes it easier for FCG parts to reach the firing pin part of the bolt, too (which is why the VZ has room for a striker)

TCB

 

[justin22885]

for the gas system i showed you, gasses wouldnt be tapped off the barrel in this location but from a normal AR-15 type gas block further down the barrel, out of this gas block would be the gas tube which would bring gasses from in front of the rifle to the gas piston.. but would it be a good idea to have the hot gasses this far rearward?.. i mean, the AR-15 sends gasses even further back so its still much better than that system, and would maintain a similar balance to the rifle by having the weight more rearward

and as it stands now i have the area behind the oprod (or area where the piston strikes if i go that route) bored out for the recoil spring and a telescoping guide rod

what i was thinking of doing was removing the rails, and drilling two holes completely through the bolt carrier behind where the rails would have been and run two guide rods through the bolt carrer and into the back side of the trunnion.. when closed the rod would be supported in the front by the rear of the trunnion, and in the back by a removable block that will hold the recoil spring too.. for field stripping you just break the action open, grab that block and pull everything out, itll likely all be captive with the carrier

does the AR-18/AR-180 use rails in this manner or do they have something attaches to the receiver that the bolt carrier rides on?

 

[barnbwt]

The AR180 is like the AR70; receiver's just a square tube the carrier rides in, and the bolt uses the floor as keying rails. The square tube attaches to a forward trunnion block, not unlike a G3 in construction, but square.

TCB

 

[justin22885]

i guess if i had the oprod stay in the trunnion, i would only need to have an extended surface contact the inside walls of the receiver to keep it centered.. so i guess with the longer oprod i wouldnt even need rails, or guide rods since between the oprod and two sides of the carrier the bolt wouldnt have any movement in any direction besides forward and back

would you agree that the walls of this trunnion look way too thin?.. especially if it were to be threaded with interrupted threads or threaded savage style barrels?.. im thinking of making my receiver thicker so to be able to have more meat in this area.. which means i either have to step up to a 1.5" piece of square tube steel with half a piece of .75" tube steel for the dust cover, or go with a 1.5x2" piece of rectangular tube steel.. or the third option.. designing a simple shape that best fits the design that can be made of sheet metal bent on a shop press and welded at the seams.. maybe dimpled on the sides to stabilize a thinner bolt carrier

 

[TRX]

Though it looks like you've moved away from the recoil-operated concept... last year I bought a Remington Model 8, a long-recoil rifle designed by John Browning. The bolt and trunnion setup look amazingly like an AK-47, except the whole assembly recoils when the rifle is fired. A big spring wrapped around the barrel, with a sheet metal shield, moves it all back into battery. (8 aficionados call it the "pogo stick").

Shortly after buying the Model 8 I picked up a PPS43 kit. While they were both on the bench, I noted that much of the exotic machine work of the Model 8 could be replaced with stamped sheet metal like that of the PPS43. You'd only need to machine the trunnion, bolt, carrier, and a few bits; the rest could be cut and bent from sheet like the AK and PPS kit builders do.

The recoiling barrel of the Model 8 gets a lot of bad publicity, but it was accurate enough to stay in production for decades. Though with a PPS-style barrel shround, you could update the design with a 1911-style barrel bushing...

The trunnion and bolt parts of the Model 8 aren't complex, and there's a lot more metal in a Model 8 trunnion than an AK trunnion, even though some AKs were made in .308 and other medium calibers. Most of the rest could be low-tech sheet metal.

 

[justin22885]

Though it looks like you've moved away from the recoil-operated concept... last year I bought a Remington Model 8, a long-recoil rifle designed by John Browning. The bolt and trunnion setup look amazingly like an AK-47, except the whole assembly recoils when the rifle is fired. A big spring wrapped around the barrel, with a sheet metal shield, moves it all back into battery. (8 aficionados call it the "pogo stick").

Shortly after buying the Model 8 I picked up a PPS43 kit. While they were both on the bench, I noted that much of the exotic machine work of the Model 8 could be replaced with stamped sheet metal like that of the PPS43. You'd only need to machine the trunnion, bolt, carrier, and a few bits; the rest could be cut and bent from sheet like the AK and PPS kit builders do.

The recoiling barrel of the Model 8 gets a lot of bad publicity, but it was accurate enough to stay in production for decades. Though with a PPS-style barrel shround, you could update the design with a 1911-style barrel bushing...

The trunnion and bolt parts of the Model 8 aren't complex, and there's a lot more metal in a Model 8 trunnion than an AK trunnion, even though some AKs were made in .308 and other medium calibers. Most of the rest could be low-tech sheet metal.

i havent moved away from recoil operation, thats a separate design im working on.. ill try to describe how this setup works for the short recoil im working on

when you close a rotating bolt, you need the carrier to rotate the bolt because the bolt face has to be against the breech of the barrel before it can begin rotating.. if you were to try to rotate the bolt by having the bolt ride inside a cammed groove inside the receiver of a typical rifle, it would be impossible to lock it on a flat surface, having a delayed blowback at best.. this of course is all talking about a typical gas operated rifle

for my blowback idea.. i intended on having the groove the charging handle rides in by angled at the end of travel, so the charging handle groove will rotate the bolt as well.. now my solution to make this work so everything locks on a flat surface is to have the bolt face and breech of the barrel contact eachother.. and then both the bolt and barrel travel together for another half inch while the bolt is turning to lock into the barrel

because the bolt and barrel travel together for a short distance youre able to turn the bolt directly without a bolt carrier and lock it on the flat surface of the locking surfaces inside the barrel.. if the two didnt travel together a short distance as stated above, this would be impossible

so the solution i worked out was that the barrel would fly back for about half an inch and stop.. some mechanism would hold the barrel rearward while the bolt cycles.. this mechanism could be as simple as a spring holding it all back.. after the bolt travels rearward and comes back to the front, the bolt face and barrel breech meet at which point the main spring overpowers the spring holding the barrel back and the two assemblies are able to travel forward together while the charging handle is turned as it travels within the groove cut for the charging handle.. this will turn the bolt, locking it into the barrel just before the two assemblies rest fully forward

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benefit of this system is that theres no need for a bolt carrier, no need for any kind of internal machining of the receiver, and the final product is a fully locked short recoil operated rifle capable of firing any caliber you decide to design it for and the entire setup is barely more complex than your common blowback submachine guns like the sten, grease gun, sterling, etc

 

[barnbwt]

That trunny does look a little thin. So you are planning on a threaded-type barrel attachment (as opposed to a pressed/pinned affair)? One thing you might consider would be making the threads external, and using a threaded collar to pull the barrel journal into the action (like an AR or Savage rifle, IIRC). That way, the expensive part --the trunnion-- can be a little smaller, and the cheap non-load-bearing collar can be made from Aluminum or low-grade steel.

TCB

 

[justin22885]

theres a couple ways im considering the barrel/trunnion setup.. one was to use a steel trunnion, locking lug surfaces machined into the trunnion using savage 10 barrels which are threaded on by hand, and then the collar is tightened down over top

another way i was considering it is having a barrel possibly pressed and pinned into a barrel extension, of the locking lugs could be machined directly into the barrel itself.. this could be attached with interrupted threads and a detent to lock it in place.. kind of like a steyr AUG barrel.. then the trunnion could be made out of lighter aluminum

the third option again is with the lugs in the barrel or barrel extension, attached in some other manner.. not sure if a lever would be enough where youd push a barrel in then turn a lever which would have a semi-circular shaft that would lock the barrel in like a pin.. or screw the barrel in from the sides like on the FN SCAR which uses four screws to hold the barrel in.. problem i have with AR-15 barrel nuts is the barrel nut is very large, takes up a ton of space and forces the gas system to be raised high enough to get around it

so... what im leaning on is either a steel trunnion with a savage barrel mounted to the trunnion, locking lugs in the trunnion, or using an aluminum trunnion with the barrel or barrel extension mounting inside the trunnion via the interrupted thread for an even quicker, simpler barrel change

with the removable bolt head, interchangeable magwells, having an easy, simple way to change barrels would allow this rifle to function well as a multi-caliber rifle

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the other way im loking at the rifle is this.. if my upper receiver is just a piece of tube steel it would be very easy to have barrel fixed to its own upper receiver.. magwell could likely just be riveted or welded on to match the caliber of the barrel installed and just swap out the BCG and lower receiver to change calibers

you know what i could do?.. i could make an extension of the trunnion like you see on the front of the AR-15. but design my own collar to go over top of it.. likely using an interrupted thread.. downside is youd still need a separate barrel nut for each barrel.. but it does look like im going to need to make my trunnion a bit larger leaving me with three choices.. square tube steel with a dust cover, but using 1.5" tube steel, i could use 1.5x2 inch rectangular tube steel.. or design a folded sheet metal upper thats simple enough to fold with a shop press and some basic dies

by the way.. the inside diameter of that trunnion where the barrel fits into is 1" in diameter

 

[justin22885]

anyway, to try to better organize what im trying to say here is that there are two areas that im currently stuck on and not 100% certain which decisions to make here.. the gas system, and the barrel attachment mechanism

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of the gas system my choices are these

1. FAL style short stroke piston
2. M-14 style system with an oprod and piston built into the gas block
3. AR-15 gas block with a "key" machined into the front surface of the oprod for the gas tube
4. short stroke piston built into the trunnion, an AR-15 gas block and gas tube will bring gas from a forward location and divert it rearward to the piston.. adjustable AR-15 blocks can be used

options 1 and 4 will require me to NOT have an oprod on my carrier and therefor some kind of internal rails must be used.. possibly two large guide rods mounted between the rear of the trunnion and the block retaining the recoil spring in the back of the carrier

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my options for barrel attachments are

1. thread the trunnion, use savage 10 barrels and barrel nut, requires headspacing between changes, locking lugs in the trunnion
2. locking lugs in the barrel, either threaded on by hand until a detent locks the barrel in place or an interrupted thread is used to do the same thing but easier and in less time
3. trunnion extends forward with the threads on the trunnion, uses a threaded barrel nut or interrupted thread barrel nut and a detent to lock the barrel in place
or 4, im not even sure if this is secure enough, or would hold zero well but the barrel is pushed into the trunnion by hand, not pressed, and a hole is drilled along the edge of the barrel to drive a pin through, or a semicircular shaft mounted on a lever

another idea i have is to machine a few lugs inside the front of the trunnion of the rifle, then machine some grooves onto the bode of the barrel so the barrel could be placed into the trunnion, and given a little twist to lock in place

 

[barnbwt]

Of your options:

1) Short stroke is most useful on open top receiver guns (VZ58, SKS, FAL) since there is no piston rod in the way as the action opens. If your design has no need for such, the only real benefit to this system is ostensibly less felt recoil

2) My main beef with the Garand/M14 system is how complex and convoluted the line of action from piston to bolt is. It's always astounded me it works as well as it does since that curvy rod has to be flexing and bowing like crazy when loaded up by gas pressure. Believe it or not, an mere AK47 is functionally very close to this system in operation, but putting the gas tube up top allows everything to be straight (and the beefy bolt carrier is just to hold the bolt instead of a complex receiver forging)

3) If an AR15 bolt will not function with a clogged/blocked gas key since the small tube won't push on the carrier hard enough across such a small area, can your piston be sized large enough to operate and remain compact? Also, you'd obviously have to do a better job than Stoner did making sure your bolt carrier can't tilt from the off-axis thrust, but that's an issue intrinsic to all piston actions. My main point about the gas tube is that a long skinny tube is incredibly inefficient in telegraphing pressure and mass (gas volume) which you'd need for a piston operation. You might as well use a full-on gas tube (and if made for a very common tubing size, any cost/availability gains from using the AR tube would be more than regained)

4) Same possible issues as number three. Instead of blasting a little tube's gas at the piston face, which is sure to be a lossy and leaky affair, you might just use a 'remote gas block' at the front of the trunnion to retain the piston, align/secure the tube, and seal the gas delivery until the piston uncovers vent holes. I'm not sure how much you end up saving over a longer piston or normal-style gas tube by using the AR tubing. A hollow pipe piston rod would weigh exactly as much as a hollow pipe gas tube, and infinitesimally more than a slightly-smaller diameter AR gas tube that's more complicated due to bends and swaging.

For barrel attachments:

1) I'd avoid this; it's been done a zillion times and isn't very interesting from a design standpoint . Barrel extensions are really the wave of the future; there's precious few advantages to putting the lugs on a larger separate trunnion
2) This is an easier to make solution. Do look into the BREN barrel attachment method; very clever way to do fast-change barrels (it's an interrupted thread collar attached to the carry handle for leverage)
3) This will allow a smaller trunnion diameter than number 2, but will probably end up being a larger package when you count the collar diameter. If the collar is no fatter than the rest of the rifle, there's very little reason to go with an internally threaded trunnion over this
4) For a barrel with integral lugs in an extension, this would probably work, at least depending on what levels of recoil you end up with (the barrel thickness at the chamber and pin diameter would need to be sized for max recoil, of course). I think this is sort of how the ARX160 works, but they have a vertically sliding block that secures the barrel/extension (not unlike the takedown levers on Beretta pistols)

Your last idea is very similar to how the Suomi attaches its barrel shroud (long story; the Suomi shroud is attached as firmly as any barrel, and itself acts as a collar to hold the barrel into the trunnion. It's a big reason the guns are so accurate). The nose of the receiver is four large square lugs, which align with mating lugs in the shroud. A simple locking lever swings up and blocks the shroud from rotating backward and coming loose. While it works great, the drawback is that the force required for every moving part is extremely high, since you don't have the mechanical advantage of threads helping clamp everything together. Part of it is machining tolerance making some shrouds much tighter, but even the loose one' require strong hands to put together.

TCB

 

[justin22885]

well... i think im going to go with an oprod anyway.. the extension of my bolt carrer you see sticking out the front.. i dont think i will use a gas tube to bring gasses back, theres really not much added weight to be saved here to make it worth it.. so from this oprod i will either have a piston mounted on the end of it that will go inside the gas tube, or i will have a small short-stroke piston head inside the gas tube

now.. what would the advantages to having the piston head separate be over attached to the bolt carrier?.. does seem like there would actually be much of a different in function here.. does a short gas piston provide any benefit thats worth the added complexity and higher parts count?

since im going to go with the op rod, its going to provide a certain level of stability to the bolt carrier.. and make carrier tilt very unlikely.. and it also means i dont really need any rails either.. the oprod prevents the carrier from tilting forward or back, or side to side and the edges of the carrier itself will prevent it from rotating

also.. i intend to put the recoil spring on a telescoping guide rod, the entire spring will take up most of the interior volume of the hollow oprod, like on the AK which should allow the BCG to get closer to the back of the receiver and allows me to shorten up the rifle

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so.. either i will go with something like that BREN used, piston head attached to the oprod attached to the carrier goes inside the short gas tube.. gun fires, carrier recoils back and the piston exits the gas tube and the gas vents.. i could bell the mouth of the short gas tube to make it easier for the piston to re-enter the gas tube after it comes back

or the other option, installing a short little piston in the gas tube like on a SCAR

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now i just need to figure out what i will do for attaching the barrel and im considering doing what the steyr AUG and also i believe what the suomi M31 does.. or going with a simple interrupted thread with a lever or a detent to hold it in place but first, at least for my prototype to keep it simple i will have my locking lugs in a steel trunnion set up for savage 10 barrels.. sure ill need to headspace whenever i change a barrel, but thats not a problem

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one more question though.. the lower part of the trunnion where the barrel is mounted.. do you think it would be possible to press a threaded bushing into an aluminum trunnion?.. the bushing would be wider on the breech end, barrel threads into it from the muzzle side of the trunnion and it locks down with a barrel nut?

 

[fletcher]

do you think it would be possible to press a threaded bushing into an aluminum trunnion?

It's possible, but there are some things that come to mind as a result:

- Galvanic corrosion between the bushing and trunnion. More of a long-term issue, but appropriate protection would be required.
- Stress corrosion of the trunnion. Could result in catastrophic failure after some time due to static stresses resulting from the press fit. Many aluminum alloys are susceptible, and this has been a documented mode of failure in service. Proper aluminum alloy and heat treat condition selection would be required.
- Balance the amount of interference such that endurance limit at a particular life isn't exceeded with the addition of tensile stresses at the hole due to the press fit.