1/2 scale AR-15

[MachIVshooter]

come up with something better than 'mini-15' also; I can't do any better at the moment

AR-7.5

when you go to shape the bolt body, please carve up the breechface to look like a miniature Stoner Sprocket. A mini gas key would be neat as well, but you'd only see it upon disassembly.

I intend to try, although the "lugs" will probably be pretty shallow, as the face will already be disproportionately large to house the .17 HM2 rim. It will have a faux gas key; gotta have something for the CH to grasp.

Even low-pressure 22LR needs a 1/3rd lb or so bolt, and 17 will need a good bit more. Have you run numbers for how heavy a bolt you'll need, and how much bolt volume you likely have to work with? I guess at worst you can always attach a 'piston' rod to the gas key that extends forward under the handguard to gain some weight in an unseen area.

A 4" long hunk of basically solid 4140 PH or AR400 (haven't fully decided yet) will have a decent mass to it. There are, of course, .22's that use much lighter slides and bolts than that of the 10/22, Marlin 60, etc. I'm sure it'll take some playing to get it right, but I'll start heavy on the springs; would rather the cartridge doesn't eject than have a case head blow out.

 

[jmorris]

Only if you already had the CAM file. CAD/CAM is very painstaking.

To learn it, this is correct. Once a guy knows what he is doing, it is faster than one could draft it on a sheet of paper. I am not that guy (yet) but know a few that can create a file faster than a kid on a cell phone. Pocketing and radius is where CNC really shines run times are a lot faster than by hand. Well you know it can't take too long to machine one or they wouldn't undos cheap as they are for a full sized one, anodized and engraved with all costs and profit included.

18 hours on a lower and trigger, my hats off to you and good work.

 

[MachIVshooter]

Pocketing and radius is where CNC really shines run times are a lot faster than by hand.

Symmetric radii (to include pockets, such as bearing pockets) are not difficult at all on a manual machine if you have a decent rotary table. I run a Troyke DMT12 compound cross slide table, Which makes it not only fast to center the work piece, but also to move from hole to hole with precision to 0.0005". Not mine, but representative:



Now, when you start getting into swoopy lines or convex protrusions, that's where CNC becomes not only far easier, but sometimes almost necessary. Trying to do fancy lettering or wacky contours on a manual machine is an exercise in extreme aggravation, and is why those detailed things were often done by hand in pre-CNC days, and still are by home machinists like me who do not have the room, money or power supply to run a VMC. I could buy a smaller "low speed" VMC with a bunch of CAT40 tooling for a reasonable price these days, but these machines have a large footprint, and I would need to bring in 3 phase, as running a rotary phase converter on 100A service can't even begin to keep up with a 15+ HP 3P motor. Then I would also have to buy Solidworks or some other program.

One day I will pick up a CNC, but I so enjoy running manual machines, I'm in no rush. I'd much rather be turning cranks than clicking a mouse.

 

[MachIVshooter]

OK, I had a little time this afternoon, so got started on the upper. I'm a little annoyed that the reamer was not the correct .465", but oh well; I can just as easily work with a .490" bore.

Anyway, one member had asked for video of a process, so rather than setting up a tripod, I decided to just film a few moments of something I could do one-handed. Nothing spectacular, just a pass on the rotary table cutting the receiver where it will be threaded for the barrel nut:

https://www.youtube.com/watch?feature=player_embedded&v=d6lPSfnZ-34

Threading the upper in 3/4-28. I centered the quill over the hole, then used down pressure of the quill with the piece dead square in the vice to make sure the threads were cut true to both axis:





And one side roughed out:



A tedious process, using a .105" ball end 2 flute mill and moving over/changing DOC by just a few thousands per pass, varied based on what part of the profile. This kind of thing is where a CNC kicks butt, but it can certainly be done manually. Rather than do hundreds of passes, I just do a few dozen, then knock the edges down by hand.

 

[barnbwt]

"I'm sure it'll take some playing to get it right, but I'll start heavy on the springs; would rather the cartridge doesn't eject than have a case head blow out."
Mass is far, far more important than spring weight in high pressure chamberings; as strong a spring as you can humanly cock back is still a drop in the bucket compared to the bolt thrust of something as weak as even 22LR (granted, stiffer springs are heavier, so you get some benefit that way ). A really easy way to get the weight right, as there's a good chance your first guess will be off on something as 'spikey' in pressure as 17HMR, is to design the bolt to be adjustable. Leave a threaded stud or hole on the back face, so you can thread on rods of differing lengths to get what is needed. A lot of folks have adjustable charging handle knob sizes, but that would look odd on this replica build.

What the spring is good for is ensuring forceful feeding (what mag do you plan on using, or will you make your own?) and preventing damage to the receiver as the bolt assembly bottoms out. I doubt either of those will be an issue for you so long as a buffer of some sort (polyurethane is a good material) is in the the tube to cushion the bolt and keep the spring from kinking, since 17HMR isn't exactly a hard kicker. Making the bolt too light will turn it into one, though, and get you ever closer to the 'danger zone' of bulged cases and et cetera.

I know Ruger did some 22mag 10/22's at some time or other, that might be a good place to get a bolt weight from if you can find people that own one. If you go too light, you'll need other tricks to get things to function, like the fluted chamber in the Kel Tec PMR30, which allows them to have a slightly lighter blowback weight without case damage.

I don't suppose you're going to keep the Picatinny rails full size, are you? Or is this an "old school" miniature build?

TCB

 

[MachIVshooter]

A really easy way to get the weight right, as there's a good chance your first guess will be off on something as 'spikey' in pressure as 17HMR

.17 HM2, not HMR

like the fluted chamber in the Kel Tec PMR30, which allows them to have a slightly lighter blowback weight without case damage.

The Grendel P30 had a fluted chamber, but the PMR30 uses a hybrid system than allows the barrel to travel rearward with the slide until pressure drops by using simple friction of case against chamber. It's not a locked breech, but not blowback either. Only gun in existeance I know of using this system.

I don't suppose you're going to keep the Picatinny rails full size, are you? Or is this an "old school" miniature build?

They will be scale at 7/16". Plan is to use a micro RDS with my own base. We'll call it mil std M956.5 pica-tiny.

 

[MachIVshooter]

OK, upper is basically done except for finish work and a couple holes. I did have to get creative with the rear take down block on account of I went a tad too far when radiusing the rear of the upper. It'll work out fine, though: having the block start further forward would only be a problem if I were an SOT making a 1/2 scale full auto M4.

A lot of profiling had to be done by hand with die grinder, dremel & files. Likewise, the finish work will be almost exclusively files and sand paper.

Anyway.........

 

[JDGray]

Awesome thread Sir!! I'm hooked!!

 

[19-3Ben]

Man, you're good.
I'm in awe of your skill.

 

[MachIVshooter]

Matched the profiles, got the charging handle slot cut, and the majority of the finish work is now done. Tricky trying to get tool marks out of those tiny nooks and crannies! Mostly done by wrapping a 3/16 screwdriver or 1/8" punch with 400 grit and painstakingly wet sanding with very small strokes.

 

[Panzerschwein]

Looks pretty good!

CAN'T WAIT FOR THE PRODUCTION MODEL!!!

 

[Field Tester]

I may have missed it, but what kind of finish are you going to put on it? Any inlay and fill?

 

[Phaedrus/69]

That is flat out amazing! You sir are an artist. Very cool!

 

[bannockburn]

Definitely a work of art in metal. Sort of like watching a sculptor carving a statue out of a slab of marble.

 

[MachIVshooter]

I may have missed it, but what kind of finish are you going to put on it? Any inlay and fill?

I was going to anodize, but probably gun kote now. Couple of small blemishes I need to take care of, and it has to be done with filler like JB Weld. Cannot weld 7075-T6.

One thing I've learned about the AR is that, while a very easy rifle to assemble from parts, it is a particularly difficult design from a machining standpoint. At least it is when you're trying to copy forged receiver halves.

 

[desidog]

One thing I've learned about the AR is that, while a very easy rifle to assemble from parts, it is a particularly difficult design from a machining standpoint. At least it is when you're trying to copy forged receiver halves.

...and this is why i get peeved when people say they "built" an AR; when in fact they just assembled one.

You can actually claim that accolade. Well done.

 

[MachIVshooter]

Still gotta make a gas key, extractor & firing pin, but the homogenous bolt & carrier is basically done. Haven't decided on ejector yet. Not sure if I want to do a plunger style like a real AR, or machine a slot in the carrier and install an ejector in the upper. Anyway:








The steel I chose for this is probably harder & stronger than a real 8620 bolt carrier; it began life as a flat air hammer punch. Not only tempered to deal with tens of thousands of rapid impacts against hard surfaces, but also hard enough to deform minimally. About the only thing that hurts these punches in normal use is bearing races. I did goof my angle on the first forward assist cut, but I wasn't about to start over; this piece of steel ate six carbide cutters and about two dozen drill bits.

The faux locking lugs are aesthetic only; they will not engage anything in the barrel. I may, however, machine a hardened steel sleeve that the shoulder of the carrier comes to rest against to avoid this tiny, aggressive bolt face beating the crap out of the breech face.

Also got a magazine figured out for now. Slightly modified Savage rimfire mag fits the well and allows the bolt to strip cartridges. Still have to make a mag catch, but I think it's a decent fit:

 

[HisStigness]

This is one of the most ambitious and impressive things I have seen in a long time. I'm assuming this is going to be straight blowback since you aren't using the locking lugs. I forsee there being some T&E getting the right buffer spring for the gun to cycle properly.

 

[JDGray]

Did you plan on using a Savage magazine, and build the lower to fit it? Or is the mag well a true 1/2 scale?

 

[MachIVshooter]

I'm assuming this is going to be straight blowback since you aren't using the locking lugs. I forsee there being some T&E getting the right buffer spring for the gun to cycle properly.

Yeah, that's the plan. I expect some time and frustration getting it to feed/fire/extract/eject reliably, but it'll work out eventually. If straight blowback proves impossible to work, I may employ a hybrid system like that of the PMR30, except that it would be a chamber that slides rearward a short distance within the barrel, rather than the entire barrel. Would probably be a fairly high maintenance system with carbon build-up, but I think doable. Still, I'm hoping I can make it work with the weight of the bolt carrier and a buffer made either of lead filled steel or pure tungsten.

Did you plan on using a Savage magazine, and build the lower to fit it? Or is the mag well a true 1/2 scale?

I think I'll be using the Savage magazines to actually fire the rifle, and make a scale mock magazine for display.

The mag well is true 50% scale (haven't profiled the step in the rear yet), but the modified savage magazine fits within it. Plan is to TIG shims to the sides of the mag to deal with the 0.084" width disparity.

I'm not sure if I'll be able to employ a working last round BHO with these mags, but I'm gonna try.

On a different note, I appreciate all the compliments, and it does take some skill & experience to pull this stuff off. However, skill alone will not make it happen. Substantial equipment and tooling is required to make this sort of thing. What I have accumulated to do these projects is well beyond what even most gunsmiths have access to. In addition to the usual vises, pedestal grinders, die grinders, dremels, band saws, hand drills, etc, the more specialized equipment includes (but is not limited to):













Machining as a hobby is a substantial time, money and space investment. I enjoy it tremendously, but it's easy to see that it requires more than a fleeting interest to acquire the necessary skills and tools. I've been into metal fabrication since I was about 12 years old, having spent my allowance & lawn mowing money at that age on things like bench grinders and mini drill presses instead of video games or the latest sneakers. I'm 32 now; you're looking at 20 years worth of investing, in addition to the support provided by my full automotive shop and associated tools of which I've made a career.

Luckily for anyone who is interested, it's more affordable than ever these days. The big machines are still spendy, but thanks to eBay and Craigslist, one can buy used end mills, reamers, etc in lots for pennies on the dollar. I've scored good quality cutters for as little as 3% of their original price. Had I bought everything new, the end mills alone that you see in the fifth picture would have cost several thousand dollars. Used to have to get lucky with yard or estate sales. Now, these things that used to sit in boxes for decades due to limited market pop up for sale right at your desk top or on your mobile.

 

[barnbwt]

".17 HM2, not HMR"
Ah, that's a fairly significant distinction (and one I wasn't even aware of, having little interest in rimfire --I always figured they were competing brand names)


Bwah-hah-ha! It's so little! You need to grab an AR10 BCG and do a Russian Nesting Dolls lineup thing with them (like the famous photo lineup of the RPD, DP28, and DShK )

"The Grendel P30 had a fluted chamber, but the PMR30 uses a hybrid system than allows the barrel to travel rearward with the slide until pressure drops by using simple friction of case against chamber. It's not a locked breech, but not blowback either. Only gun in existeance I know of using this system."
Wow. Remind me to temper my "there's no comparison between the five-seven and the PMR30" next time someone tries to; the FNH gun employs this exact mechanism to 'cheat' down their slide mass a hair, though it requires Teflon coating on a cartridge that powerful to avoid tearing/rupture. Is there any design Kel-Tec makes that isn't ripped off j/k (but not really )

The 'sliding chamber' idea you describe is also known as a 'floating chamber' and is a cheap way to get sub-caliber conversions running reliably. When fired, the cartridge pressure acts on the sealed breechface, but also the whole frontal area of the floating chamber element that is separate from the bore (at least, once it begins to recoil). This has the effect of a recoil booster on the reciprocating parts, enabling a little bitty 22LR to cycle a big honkin' 1911-sized slide authoritatively. I believe the Pedersen Device SMG conversion worked similarly. Seeing as your bolt looks in the ballpark of what we expect from 10/22 type actions, I suspect you won't need such drastic measures to achieve function (I suspect this might be why you didn't do 22WMR? Or was this really just a happy coincidence?)

As far as ejector setup, I would avoid a push-button type. You already have an offset rimfire firing pin drilled through carefully that will 'look' like the ejector, so it may be worth considering something else before plunging teeny bits into hardened metal right next to also-teeny holes. Here's a wacky idea; a wire or leaf spring mounted in the upper along side the bolt body which will spring outward and knock the round out the port when the bolt is fully back, then is pushed back out of the way by the bolt as it returns. Totally unseen unless the bolt is pulled back, and requires no delicate (or fragile) parts/mods to your perfect bolt body . As a backup, I'd just cut a fixed-blade ejector slot between two of the lugs and call it a day.

Maybe my eyes are deceiving me, but it looks like 17HM2 is just slightly undersized 'to scale' for your mag well length. I wonder if you might have an easier time making a functional AR15 lookalike mag than you think; if the round is short enough, you could mill separate 'filler' pieces to make up the front and back walls of the mag, with side plates that attach with tiny little screws. If you can transfer the interior of the Savage mag into the inside of the 1/2 scale AR15 shell, you'd have a bolt-together, functional mag that is easily made on your CNC.

TCB

 

[MachIVshooter]

Here's a wacky idea; a wire or leaf spring mounted in the upper along side the bolt body which will spring outward and knock the round out the port when the bolt is fully back, then is pushed back out of the way by the bolt as it returns.

A possibility. IIRC, that system was used in the Remington 24/241.

you'd have a bolt-together, functional mag that is easily made on your CNC.

No CNC here, brother. 100% manual equipment, with DROs to make things a little easier (reading dials and having to account for backlash whilst keeping an eye on your part just plain sucks).

Can do pretty much anything on the manual that you would on a CNC, but some things are insanely time consuming. Milling an AR-15 magazine profile is one such thing; it's a progressive radius. Not too awful with a sine bar, but I have no such piece of equipment at this time. So, what ends up having to be done is moving X & Y alternately by changing ratios as you progress along the curve. These movements are literally as small as the readouts allow: 0.0005". I have done it, and it takes FOR-EV-ER. Seriously, I mean it could take an hour to cut a 3" long arc. It's also really hard on cutters to move that slowly and dwell so much-even in aluminum.

 

[davidhan]

This is one of the best threads I've seen in a long time. I saw a 1/4 scale lower someone made when they first setup their CNC. But this 1/2 scale is way cooler and will be functional to boot!
keeping my eye on ya!

 

[MachIVshooter]

Grip done. Well, almost; I need a more course bastard mill to make proportionate checkering.

Milled from solid black ABS

 

[Catshooter]

MachIV,

Very nice work!

If you haven't seen the inside of a Kel-Tec PMR 30 .22 Magnum magazine, you might want to. Very ingenious design to handle the rimmed cartridge.


Cat