Why Has No One Else Used the AR DI System?

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I used to have a really nice Costa Mesa AR-180 that I regret having let get away. I've always thought that it was a better basic design for a general issue infantry rifle than the AR-15. It has almost all the AR-15's virtues (e.g. light weight, superior ergonomics, modularity [potential in this case, as the weapon was never developed like the AR-15 was, but there's no reason it couldn't have been]), and eliminates its one (arguable) vice -- the way the quasi-DI system of the AR-15 pumps hot gas and carbon fouling into the receiver. True, the AR-18 probably doesn't have as much potential for gilt-edged accuracy as the AR-15, but for a general issue infantry rifle, that really makes no difference whatsoever; the AR-18 had accuracy to spare for that role. All the flaws the AR-18 had -- it's rather flimsy folding stock, the less robust (stamped) fire control parts -- could have, and would have easily been worked out had the rifle undergone the kind of product improvement in service that the AR-15 did.
thats my opinion of the AR-18 as well, and they would be so easy to mass produce in a small shop as an expedient, yet still modern, fully functional military rifle, i wish it had even half the development the AR15 had
 
I think you guys are splitting hairs as to what a DI system is. It's like saying a piston system can only be of one type. The AR is a type of DI system.
 
Work on your reading comprehension. I said bolt design; the similar (not identical) lug pattern is the only thing they have in common. No separate bolt & carrier in the Johnson, totally different camming mechanism, different extractor, different ejector, etc. Saying that the lug design makes it a facsimile means you just as well call anything with two opposing lugs an 1886 Lebel ripoff.

Already gave you that I erred on the barrel extension, noting that they are also similar but not identical. I know to admit when I've made a mistake. You need to learn to do that.



Forget "not a true DI system"; it's not a DI system at all. It's a piston system without an op rod.
Well, guess who was a consultant to Armalite in the early 1950s? None other than Melvin Johnson.

Yes, Armalite copied Johnson's basic bolt and barrel extension design. With good reason, the design is simpler to make than say an M1 or AK bolt. Both can be made with a simple turning operation followed by a simple broaching operation
 
Well, guess who was a consultant to Armalite in the early 1950s? None other than Melvin Johnson.

Doesn't change any of the facts in that statement, though, does it? Still no carrier for the Johnson bolt. Still different camming mechanisms. Still no gas rings on the Johnson bolt. Still different extractor & ejector design. Some would, in fact, consider them quite different altogether.

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I think you guys are splitting hairs as to what a DI system is. It's like saying a piston system can only be of one type. The AR is a type of DI system.

No, it isn't. It is a gas piston system with no operating rod, one that uses a stationary piston and moving cylinder instead of the conventional stationary cylinder and moving piston attached to a rod. This is clearly outlined in Stoner's patent:

http://www.google.com/patents/US2951424
 
No, it isn't. It is a gas piston system with no operating rod, one that uses a stationary piston and moving cylinder instead of the conventional stationary cylinder and moving piston attached to a rod. This is clearly outlined in Stoner's patent:

http://www.google.com/patents/US2951424
in that case, my MAS 49/56 isnt DI either, its a gas piston as well its just the gas piston and the bolt carrier are the same piece with no oprod
 
"The number of rifle designs that do, or do not, use a particular operating system (independent of number of rifles fielded/applications for which the rifle is used/history or length of time the rifle has been in use/etc.) is a deeply weird metric to hang your hat on."
And yet the absence of designs using a feature is frequently cited as evidence of their poor utility. The number of rifles made in excess by a corrupt economic system (AK/SKS/Mosin), or given away for diplomatic favors to shadow conflicts (AR/FAL/Garand) is likewise a weird metric to hang the hat on.

"you see far, far more adaptations than the AR-18 (based on stoners AR-16) than anything else, no one else has put the AR gas system into a new rifle, a few but not that many have been adopting the AK gas system, but many, many rifles use a carrier and short-stroke system similar to the AR-18"
Exactly. The cutting edge is hardly an indicator of 'the best design,' but it is a solid indicator of where the best minds in the industry believe improvement can be found, and they are very nearly all in agreement on this sort of piston system (the CZ BREN and Tavor I think are the only non short-stroke designs for 223/308 size platforms chasing military contracts at this time, which aren't direct clones of earlier designs like the AK/AR/etc)

"If you copy the DI you are going to end up with something much like an AR anyway"
No reason at all a similar scheme could not be used on an AK bolt/carrier if the spring were relocated, yet no one bothers to even try as an experiment. No reason at all the convenient overhead conduit in an HK roller gun couldn't be used in conjunction with a 180deg locking piece (i.e. not a wedge) to yield a DI gas operated HK --yet to my knowledge the gas conversion has only been done with pistons. Basically, there's no reason at all any telescoping bolt/carrier arrangement (even one employing a third locking piece like the VZ58) could not operate on the 'expanding gas inside the bolt' concept while retaining other unique features...and yet there's not one design I've heard of, past present or proposed for the future, that's even bothered to try.

Whether it's the obvious heat implications for the most critical load-bearing parts, the terrible powder fouling whether impacting operation or not, or just the fact that the spindly gas-tube you need to actually reap those vaunted weight savings so frequently bandied about will overheat and rupture long before any competing gas piston system, something is keeping designers from keeping the baby (direct impingement) when they (now infrequently) care to toss out the bathwater & do a fresh design.

There have been more designs trying (and failing) to use primer actuation, than there have been DI systems with any resemblance to the AR/Stoner system. It's just as weird a system, as far as its specific influence on others' design ambitions, as the Madsen LMG's recoil operation system.

And the folks who keep thinking that because the AR's bolt, receiver, or stock designs get copied elsewhere for their obvious utility, need to quit pretending that has anything at all to do with the pros/cons of direct impingement. All those features can be added to any other design independent of each other, and have been...

...except for the gas system. :)

TCB
 
"in that case, my MAS 49/56 isnt DI either, its a gas piston as well its just the gas piston and the bolt carrier are the same piece with no oprod"
It's pretty bad when the primary defense of DI's oddity is that DI is actually a piston system like all the others. Just sayin' ;)

Some sort of Poe's Law thing going on in this thread :D

TCB
 
According to patent, direct impingement is not the proper name for the Stoner system, it is an expanding gas system which is different. Reading the patents makes one realize the genius and innovation this rifle was at the time. Keep in mind this was designed as a military weapon capable of full auto operation. Which proved more practical when paired with the 5.56 NATO.
The main problem with doing another type of DI system is where you put the recoil spring. If you put it in front of the action like a 7400, you need action bars so you might as well impinge where the 7400 does. To build a convention looking rifle the gas system would have to be on the left side of the receiver and action spring angled down the buttstock. Possible but awkward. The design as a whole is hard to improve on from a practical standpoint.
That is some many new rifles are copies of the Stoner system. It is a really good system as a whole.
 
Whether it's the obvious heat implications for the most critical load-bearing parts
I'm not aware that the bolt head gets significantly hotter in an AR than with an AK, or that bolt failures from overtemps are common on AR's. Do you have a link to a source with comparative temps, or discussion of bolt-overheat-failures on AR's?

the terrible powder fouling whether impacting operation or not
In my experience, an AR doesn't foul the receiver any worse than an AK (which blows gas down the gas piston tube into the receiver, and blows gas out of the chamber when the bolt opens just like an AR). Most receiver gas in a semauto, AR or not, comes from the opening chamber; the AR vents most of its operating gas out the gas vents, not into the receiver.

the fact that the spindly gas-tube you need to actually reap those vaunted weight savings so frequently bandied about will overheat and rupture long before any competing gas piston system
Cite? If you are shooting a government-profile barrel, the barrel will droop and burst before the gas tube fails. And unless you are shooting full auto, you will never get the gas tube hot enough to fail; you'll shoot the rifling out of the barrel first. Source: The M4 vs. M4A1 test-to-destruction series (I can dig up the link to the vids if you want).

You could also go with a thicker walled gas tube if you feel the need, but I'm not aware of any civilian gas tube ever blowing out (or any military tube outside of extreme test conditions, not even at Wanat), unless somebody jammed a foreign object in it.

something is keeping designers from keeping the baby (direct impingement) when they (now infrequently) care to toss out the bathwater & do a fresh design.
Again, if you replicated an AR's gas system, and replicated its other good features (straight-line recoil, lockup on the barrel extension rather than the receiver, modularity), you'd essentially have another AR. Since parts commonality and modularity is also a selling point, it makes little economic sense to not make an AR variant unless you want to be really different or market to the anterior pistons uber alles segment, or are designing your system for a very short barrel, suppressor, or folding stock, which are features the AR design doesn't work as well with as a forward piston and op-rod might. For a 14.5" to 20"+ barrel, and unsuppressed, the Stoner system is an excellent combination of accuracy, light weight, and reliability, though.
 
I'm not aware that the bolt head gets significantly hotter in an AR than with an AK, or that bolt failures from overtemps are common on AR's. Do you have a link to a source with comparative temps, or discussion of bolt-overheat-failures on AR's?....
AR bolts have a relatively short life because the bolt lugs are on the hairy edge of being to weak.

There are two major reasons for this: 1) the AR-15 bolt head was designed before there was easy access to stress modeling*, and 2) the chamber pressure has increased almost 25% above the original pressure.

_____________________________
*Rudimentary stress modeling could be done, the Germans did it when they were designing the MG42. They just had a room full of guys with slide rulers toiling away for a few weeks.
 
And the folks who keep thinking that because the AR's bolt, receiver, or stock designs get copied elsewhere for their obvious utility, need to quit pretending that has anything at all to do with the pros/cons of direct impingement. All those features can be added to any other design independent of each other, and have been...

...except for the gas system.
As benEzra pointed out, most of your objections to the AR appear highly theoretical -- based on what you think probably would happen with hard use; not anything you can cite any evidence of actually having happened.

As for the fact that the not-quite DI system of the AR being copied nowhere else, what of it? You seem to be implying that indicates its inferiority. This is not necessarily so. Early on in the development of new technologies, all manner of different systems get tried. Different inventors come up with different ways to do the same job, and patent their inventions. Other inventors have to work around those patents, and come up with something radically different. Look at semi-auto pistols, for example. Starting around the end of the 19th century, different inventors began experimenting with them, and had to come up with ways to lock the breech closed until chamber pressures dropped sufficiently. Toggle locks, locking blocks, rotating barrels, tilting barrels, delayed blowback, tipping breechblocks, and more were all among the systems tried. Eventually, all the practical methods for locking a handgun breech get tried and later designers select from available options (usually the relevant patents have expired by this point.) Then the design engineers make trade offs and weigh the costs and benefits of whatever systems they may be thinking of using. In time, the vast majority of handgun designers settled on the Browning tilting barrel as having the best combination of reliability, long-term durability, and economy of production, so the overwhelming majority of semi-auto pistols now use this system. This doesn't make other systems inferior. The Beretta 92, or M9 as it's known in the US military, uses a different system (a locking wedge copied from the one in the Walther P-38), and works extremely well. There's nothing wrong with it. In fact it outperforms some pistols that do use the Browning tilting barrel (it did so to win the US pistol trials in the mid-1980s), despite the fact that no one has copied that system since then.

Same with the AR and its particular gas system. Other designers have found it more desirable for one reason or another to use a gas system cribbed from the AR-18. Probably reasons of economy of production are the overriding reason, but who knows? This doesn't mean the AR-15's gas system is a deficient design, or that the AR-15 is not one of the world's best military rifles. It is. And I say this as someone who bought all the anti-AR hype for years, until using M-16s and M-4s in the army, and owning my own ARs in the years since proved to me beyond any doubt that it's one of the best rifles of its kind money can buy, and these days, far cheaper than most other weapons in its class.
 
AR bolts have a relatively short life because the bolt lugs are on the hairy edge of being to weak.

There are two major reasons for this: 1) the AR-15 bolt head was designed before there was easy access to stress modeling*, and 2) the chamber pressure has increased almost 25% above the original pressure.

_____________________________
*Rudimentary stress modeling could be done, the Germans did it when they were designing the MG42. They just had a room full of guys with slide rulers toiling away for a few weeks.
ive mentioned before perhaps on this forum or maybe on another the issues with the multi-lug bolt design and while in theory, or even sketched out on blueprints its not that bad of an idea, in practice its not a very good one at all

machining, even CNC is not 100% accurate and unless every single lug on the bolt as well as the trunnion are 100% perfect (impossible) then 3-4 lugs will end up taking the majority of the bolt thrust forces. if all pressure was evenly distributed amongst all lugs the AR-15 bolt would be strong enough to handle .308, but it isnt for the simple fact we dont live in a perfect world.. thats why i think a proper rifle bolt should have no more than 4 lugs, 2 or 3 being better and a two or three lug bolt despending on shape can even be easier to machine... the downside is it requires a longer cam groove that probably wont work well with the carrier design in most modern rifles

my favorite bolt design ever produced is that of the FG-42, later adopted in the M60 as well as the tavor which placed the cam groove on the body of the bolt instead of the carrier
 
As benEzra pointed out, most of your objections to the AR appear highly theoretical -- based on what you think probably would happen with hard use; not anything you can cite any evidence of actually having happened
Of course they're theoretical; no one else ever copied the system, ergo my 'examples' are hypothetical. That's the point. I've never even heard of any prototype designs using a similar principle (gas inside the carrier driving the carrier away from the bolt) which is shocking, since pretty much every conceivable idea has been done at least a half-dozen times at some point in history.

"I'm not aware that the bolt head gets significantly hotter in an AR than with an AK, or that bolt failures from overtemps are common on AR's. Do you have a link to a source with comparative temps, or discussion of bolt-overheat-failures on AR's?"
Wait, I thought this was common knowledge & the subject of countless piston-conversion infrared comparison tests showing reduced temps on operating parts. The impact of this difference is debatable (effects on material properties or burning off of lubricant) but the bolt/carrier are simply hotter --during sustained fire-- than piston designs that keep the hot gas further away.

"In my experience, an AR doesn't foul the receiver any worse than an AK (which blows gas down the gas piston tube into the receiver, and blows gas out of the chamber when the bolt opens just like an AR). Most receiver gas in a semauto, AR or not, comes from the opening chamber; the AR vents most of its operating gas out the gas vents, not into the receiver."
This is because, like the AR, the AK's gas tube is largely vented into the receiver. Guns with tighter piston tubes (AR70 is my personal exposure) do not have the issue, nor do any of the short-stroke designs which are basically the cleanest non-recoil-operated systems there are. Please go on and explain how an AR10 vents just as much fouling into the receiver/FCG area as my FNAR which remains clean for hundreds of rounds, though. I would also recommend looking over a slo-mo of a MAS49/56, which shows that a large volume of gas is delivered through the gas tube (which is very similar to the AR, the bolt-end of things not so much), without a corresponding 'puff' as the bolt retracts.

"Cite? If you are shooting a government-profile barrel, the barrel will droop and burst before the gas tube fails. And unless you are shooting full auto, you will never get the gas tube hot enough to fail; you'll shoot the rifling out of the barrel first. Source: The M4 vs. M4A1 test-to-destruction series (I can dig up the link to the vids if you want).

You could also go with a thicker walled gas tube if you feel the need, but I'm not aware of any civilian gas tube ever blowing out (or any military tube outside of extreme test conditions, not even at Wanat), unless somebody jammed a foreign object in it."
For like the third time, I'm not saying these are issues in practice. Please stop taking this as me 'knocking' your baby. Gas tube ruptures are not unheard of failure modes on those moronic 'torture tests' garnering all the Youboob hits. Super hot gas, thin wall stainless tube, blammo. I believe the couple I saw were on heavier barrel wannabe-SAW platforms, FWIW (I agree the barrel is long toast on pretty much every automatic there is given sustained full auto). My point in raising this aspect is that the gas system of the AR is ultimately 'weaker' --i.e. more prone to bursting or failing due to overheating-- than piston designs, and this is largely due to the fact that pistons necessarily have beefier parts. If you made the AR tube as beefy as an AK/etc's, it would likely be just as durable, but also just as heavy. The piston-rod-free design of the AR allows you to shave off its weight, but it's not as though there isn't a marginal tradeoff in doing so. No, it is not generally an issue, but it might explain why you don't see any LMG/HMG designs using the gas system --which is the question that is the subject of the thread.

"Again, if you replicated an AR's gas system, and replicated its other good features (straight-line recoil, lockup on the barrel extension rather than the receiver, modularity), you'd essentially have another AR."
Well, unless you aren't trying to make a carbine. For instance, the FAL (short stroke, rear-tilting bolt) has been used in the PTRS cannon, the AVT automatic rifle, the FAL battle rifle, and the SKS assault rifle, and I think the Chinese have a QBZ thing that is similar but with modern mags (and I think there's an ancient pistol-cal thingie with similar arrangement). The Goryunov (long stroke, side-tilting bolt) was used in one of the Gewere rifles if memory serves. The Degtyarev flapper-lock was used in the DShK HMG, the DP28 LMG, the RPD VLMG, and some early SMGs. The AK was scaled up to make the PSL and PKM.

The AR spans 223 to 308, with some peripheral efforts to get pistol cals running (successfully in 9mm, 45acp, and 7.62x25) as well as the 30-06/300WSM OMEN thingies. Tons and tons of development between 223 and 308, but not very much going on apart from that, certainly not as part of a serious marketable effort or different role than 'carbine.' No automatic rifles (designed for sustained fire like an M60/BAR), no mounted (let alone crew served) guns, no SMGs, no cannon.

"anterior pistons uber alles"
You say the AR gas system is so perfect that every other design today (or ever before, I assume) is simply being contrarian in the face of its greatness, despite my meticulous explanation of why its intrinsic shortcomings prevent it from being used in other designs, and I'm being the fanboy :scrutiny:

"designing your system for a very short barrel, suppressor, or folding stock, which are features the AR design doesn't work as well with as a forward piston and op-rod might. For a 14.5" to 20"+ barrel, and unsuppressed, the Stoner system is an excellent combination of accuracy, light weight, and reliability, though."
Yeah, it's kinda sucky outside this 'carbine' application, which it rightfully dominates. This is due to design tradeoffs compounding to unacceptable levels in most other design regimes. Which is part of the reason it isn't adapted for other designs' use. Especially given how many gun designs are actually focusing on the features you list specifically since they aren't served well by the AR (SBR, suppression, folding stocks, belt fed HMGs, etc)

"But, Stoner didn't. He freely acknowledged the Johnson influence on the bolt head and barrel extension."
I swear I read that Johnson was officially involved in the design itself early on as a consultant (I assume he retired shortly thereafter given what his age had to have been by then). It's a shame he never got to see the M16 perfected to vindicate his bolt design, he died probably assuming it was destined to be another turkey like his LMG design or the DROR when he passed in '65.

TCB
 
"As for the fact that the not-quite DI system of the AR being copied nowhere else, what of it? You seem to be implying that indicates its inferiority."
Well, this seems to be evidence of inferiority in literally any other attempt at technological advancement anywhere else (gyrojet, etronix, dardick, etc). The fact that the AR gas system is not slipping the surely bonds of the AR platform is primarly due to A) the platform has become hopelessly popular, so its traits can no longer be abandoned regardless of potential improvement (victim of its success), B) it's far and away good enough for the tasks asked of AR carbines, if not the best at this time, and C) it is clearly not the best of solutions for a gas system so it is not frequently borrowed when clean-sheet designs are in the offing.

"Toggle locks, locking blocks, rotating barrels, tilting barrels, delayed blowback, tipping breechblocks, and more were all among the systems tried."
The C96 locking system is chock full of drawbacks, and was obsoleted by superior designs very quickly (esp after 1910), yet they remained very popular even past the fall of China. No one was borrowing the design for new pistol or rifle designs, though. Same can be said for the BAR, sadly, which while a completely viable locking system, is truly a funky bird of a machine, yet was forced into service over simpler & equally effective designs due to the BAR's prolific service record. Same for the Garand --a first generation technology clearly improved upon-- being needlessly shoehorned into the next generation of rifles despite lessons learned clearly pointing to something like the FAL in a smaller cartridge. My point is that popularity in service is not necessarily indicative of superiority, and that imitation is the sincerest recognition of a design feature's desirability. The DI gas system is alone among the ARs featureset insofar as it's lack in this area; it is also solely responsible for the many teething issues the platform has had over the decades.

As far as the M9? Not even Beretta sees the Walther DNA as worth preserving at this point, but has embraced the Steyr-Hahn's with excellent results (PX4). Turns out the 'open top' slide wasn't really an advantage in practice, but the wider slide body and inability to adapt to various barrel/slide lengths were distinct disadvantages. The safety design and comfortable grip were keeps, though.

"AR bolts have a relatively short life because the bolt lugs are on the hairy edge of being to weak."
This is true. The AR is a bit like the G3 in that they were scaled down a wee bit farther than is generally intended in service small arms, leading to faster wear even if the gun is still very reliable. .MIL stuff is typically sturdy to the point of silliness, but no one can say this about the AR. Not a bad thing, as they 'only' shaved like half the weight of its nearest competitor while remaining tough enough with decent armorer support, but again, tradeoffs.

"my favorite bolt design ever produced is that of the FG-42, later adopted in the M60 as well as the tavor which placed the cam groove on the body of the bolt instead of the carrier"
Be advised, the FG42 supposedly had/has cam groove galling issues (self-limiting, though). The FG42 has a tubular carrier/bolt setup, right? So you could theoretically do a DI system for that platform, right? Would you? I wonder why no one is chomping at the bit to create this vastly superior system that bridges the gap between carbine and bullpup without sacrificing modern performance? /sarc ;)

TCB
 
ive mentioned before perhaps on this forum or maybe on another the issues with the multi-lug bolt design and while in theory, or even sketched out on blueprints its not that bad of an idea, in practice its not a very good one at all

machining, even CNC is not 100% accurate and unless every single lug on the bolt as well as the trunnion are 100% perfect (impossible) then 3-4 lugs will end up taking the majority of the bolt thrust forces. if all pressure was evenly distributed amongst all lugs the AR-15 bolt would be strong enough to handle .308, but it isnt for the simple fact we dont live in a perfect world.. thats why i think a proper rifle bolt should have no more than 4 lugs, 2 or 3 being better and a two or three lug bolt despending on shape can even be easier to machine... the downside is it requires a longer cam groove that probably wont work well with the carrier design in most modern rifles

Every time you try to talk about machining, you make it glaringly obvious that you have zero machining experience. You do realize that properly maintained common VMCs and millturns can hold tenths, right? And you do realize that the AR bolt is semi-floating, and that the lugs will wear into each other quickly, mitigating any minuscule tolerance differences in short order? Do you also realize that the way an AR bolt is manufactured has the rear faces of the lugs being cut simultaneously in a lathe operation? Inside face of the barrel extension is cut the same way.

As for what they can handle, well, obviously the multi lug design handles .308 just fine when scaled to the appropriate size.

I do find it interesting that you so despise the AR-15 and are really down on the bolt design, whilst championing the AR-18 that uses the same bolt minus the gas rings. How do you reconcile that?
 
"As for the fact that the not-quite DI system of the AR being copied nowhere else, what of it? You seem to be implying that indicates its inferiority."
Well, this seems to be evidence of inferiority in literally any other attempt at technological advancement anywhere else (gyrojet, etronix, dardick, etc).
Apples and oranges comparisons. The gyrojet was not a firearm, it was a completely different technology. The Dardick was a mechanical repeater that offered no advantage over the revolver, let alone the auto pistol. With the AR, we are talking about something that is simply a different variant of gas operation. It's more similar than it is different.

The fact that the AR gas system is not slipping the surely bonds of the AR platform is primarly due to A) the platform has become hopelessly popular, so its traits can no longer be abandoned regardless of potential improvement (victim of its success), B) it's far and away good enough for the tasks asked of AR carbines, if not the best at this time, and C) it is clearly not the best of solutions for a gas system so it is not frequently borrowed when clean-sheet designs are in the offing.
You are ignoring economics. Just because a thing does or does not get adopted, does not automatically prove it's better or worse. There are far more factors in play. I've already stated that I think the AR-18 was a better design, overall, for a general issue infantry rifle than the AR-15 was. But it didn't get adopted. By your logic that must mean it was inferior to the AR-15. But it was more a matter of economics. The US military was already committed to the AR-15, and there wasn't enough (if any) advantage to switching over to the AR-18. Expected foreign sales for the AR-18 didn't come, because although it had been designed specifically so as to be suitable for manufacture in relatively primitive facilities -- so that poorer nations could make them under license -- that plan went nowhere because it was just as cheap if not cheaper for countries to by AR-15s from the US. So the AR-18 failed in the marketplace, but not because of any deficiencies in the rifle itself or its design.

Like the Beretta 92's locking wedge design, it may simply be a feature that works superbly just as it was intended to, but is a bit less economical to make than an alternative mechanism, so there's no particular benefit to copying it.

The C96 locking system is chock full of drawbacks, and was obsoleted by superior designs very quickly (esp after 1910), yet they remained very popular even past the fall of China. No one was borrowing the design for new pistol or rifle designs, though. Same can be said for the BAR, sadly, which while a completely viable locking system, is truly a funky bird of a machine, yet was forced into service over simpler & equally effective designs due to the BAR's prolific service record. Same for the Garand --a first generation technology clearly improved upon-- being needlessly shoehorned into the next generation of rifles despite lessons learned clearly pointing to something like the FAL in a smaller cartridge. My point is that popularity in service is not necessarily indicative of superiority, and that imitation is the sincerest recognition of a design feature's desirability. The DI gas system is alone among the ARs featureset insofar as it's lack in this area; it is also solely responsible for the many teething issues the platform has had over the decades.
I'm not the one saying it is. YOU are the one saying the rifle's lack of popularity as an inspiration for later designs must mean it's inferior. You can't have it both ways. Appeal to popularity is a fallacy. That applies just as much to lack of popularity. Imitation is the sincerest form of flattery is an old saying, but that doesn't for an instant mean it's the only form of it. And whether you like it or not, the AR is a superb weapon, and it wouldn't have become the longest serving infantry rifle in US military history if that were not so.

As far as the M9? Not even Beretta sees the Walther DNA as worth preserving at this point, but has embraced the Steyr-Hahn's with excellent results (PX4). Turns out the 'open top' slide wasn't really an advantage in practice, but the wider slide body and inability to adapt to various barrel/slide lengths were distinct disadvantages. The safety design and comfortable grip were keeps, though.
Which just goes to illustrate my point all over again. The Walther-style locking block may be more limiting in some respects, making it harder to adapt the pistol to different calibers, different barrel and slide lengths, etc. When you're a military shopping for a new service pistol, who cares? All that matters is that the pistol be a good military sidearm and pass all the reliability tests to qualify. Does the M9. Yes! In spades.

Perhaps the constraints imposed on the pistol by the wedge locking system made it more attractive for Beretta to come up with a different operating mechanism for future designs. But that in no way whatever means that the locking wedge system doesn't work superbly for the Beretta 92. If the gun can pass all the reliability tests, and it makes a perfectly suitable military service pistol it doesn't matter if it's locking mechanism makes it less intrinsically suitable for redesign into a compact, concealed carry pistol. That may make it less versatile. That doesn't make it one jot less effective for its intended purpose.

By the same token, the AR's gas system may impose limits on the design that make it less attractive for other designers to copy it. Or perhaps their reasons are more economic than engineering-related. None of that makes the AR less effective in the role for which it was designed.
 
"my favorite bolt design ever produced is that of the FG-42, later adopted in the M60 as well as the tavor which placed the cam groove on the body of the bolt instead of the carrier"
Be advised, the FG42 supposedly had/has cam groove galling issues (self-limiting, though). The FG42 has a tubular carrier/bolt setup, right? So you could theoretically do a DI system for that platform, right? Would you? I wonder why no one is chomping at the bit to create this vastly superior system that bridges the gap between carbine and bullpup without sacrificing modern performance? /sarc ;)

TCB
im not sure if id call the FG42 tubular, no more so than a lot of other rifles, it basically has a two-chamber receiver that is essentially one tube on top of the other with the bolt riding in the upper tube which also holds the bolt in the top section, the oprod in the lower section.. its still a very reliable in-line system though the receiver could sure as hell use some simplifying

but no, i would never convert one to DI, its long-stroke system is very simple, hollowed out to fit the recoil spring and the removable charging handle is also retained by the spring so its all a very simple system that just works, so why mess with it?

"AR bolts have a relatively short life because the bolt lugs are on the hairy edge of being to weak."
This is true. The AR is a bit like the G3 in that they were scaled down a wee bit farther than is generally intended in service small arms, leading to faster wear even if the gun is still very reliable. .MIL stuff is typically sturdy to the point of silliness, but no one can say this about the AR. Not a bad thing, as they 'only' shaved like half the weight of its nearest competitor while remaining tough enough with decent armorer support, but again, tradeoffs.
the ARs closest competitor would be the AK-74 which is about 0.14lbs heavier with still a lot of room to lose some more (such as a combo gas block/front sight removing the front sight block)
 
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ive mentioned before perhaps on this forum or maybe on another the issues with the multi-lug bolt design and while in theory, or even sketched out on blueprints its not that bad of an idea, in practice its not a very good one at all

machining, even CNC is not 100% accurate and unless every single lug on the bolt as well as the trunnion are 100% perfect (impossible) then 3-4 lugs will end up taking the majority of the bolt thrust forces. if all pressure was evenly distributed amongst all lugs the AR-15 bolt would be strong enough to handle .308, but it isnt for the simple fact we dont live in a perfect world.. thats why i think a proper rifle bolt should have no more than 4 lugs, 2 or 3 being better and a two or three lug bolt despending on shape can even be easier to machine... the downside is it requires a longer cam groove that probably wont work well with the carrier design in most modern rifles

my favorite bolt design ever produced is that of the FG-42, later adopted in the M60 as well as the tavor which placed the cam groove on the body of the bolt instead of the carrier
A few things...

The multi-lug design is fine, and all of the load is distributed close enough to equal so as not to be a problem, in spite of the runout of the back of the lugs being less than 0.0005".

And no, the AR-15 bolt will not last long under .308 bolt thrusts. The AR-15 design bolt head should not be subjected to bolt thrust much more than 7000 pounds (more on that below), a proof round has a bolt thrust of around 7700 pounds. A .308 Win/7.62mm NATO has a bolt thrust of around 10,900 pounds.

If you look at AR/M16 bolts that fail due to high rounds, they almost always fail the two lugs either side of the extractor. That is because the stress on these two lugs is slightly higher that the rest due to the altered geometry from the extractor slot. In the following image the red areas are area where the stress actually exceeds the yield limit for Carpenter 158 steel. Once you start loading a piece of steel at the yield limit or above, you start to get low cycle fatigue.

Untitled_zpsmxywkzwz.png

The thing about steel is that is has what is known as fatigue or endurance limit. This is a stress level that, if loaded below this limit, will never see fatigue cracking. The M1/M14 and AK bolts have such oversized lug areas that the stress is below the fatigue limit, or close enough to it for the fatigue to be in the high-cycle regime, i.e., millions of cycles, not the thousands of cycles for low-cycle fatigue. Like I said, it was designed without the benefit of stress analysis. (You may have noticed that the early Armalite AR-10s made by Costa Mesa and Artillerie Inrichtingen have a different bolt head from the later ones, which are much beefiers. Also, the Stoner 63 bolt head has beefier lugs that the AR-15 and AR-18.

And, on last thing about multi-lug ('multi' being greater than three) bolts...most all large caliber automatic cannon have multi-lug bolts, both radial and axial lug layouts, some have bolt radial and axial lug layouts.
 
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"But, Stoner didn't. He freely acknowledged the Johnson influence on the bolt head and barrel extension."
I swear I read that Johnson was officially involved in the design itself early on as a consultant (I assume he retired shortly thereafter given what his age had to have been by then). It's a shame he never got to see the M16 perfected to vindicate his bolt design, he died probably assuming it was destined to be another turkey like his LMG design or the DROR when he passed in '65.

TCB
Well, Armalite paid Johnson royalties on his patent.

In 1965, Johnson, only 56 years old, had not retired from the firearm business, but he had retired from the Army reserve*. He died from a sudden heart attack. The AR-10 had shown that his design worked well and the M16 was just being fielded in large numbers.

________________________
*Yes you read that right, while every where you see "Melvin M. Johnson USMCR, but, in 1949, Johnson transferred from the Marine Corps Reserve to the Army Reserve with an appointment as Colonel in the Ordnance Corps, a rank he held until retirement from the Army Reserve in 1961.
 
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If you look at AR/M16 bolts that fail due to high rounds, they almost always fail the two lugs either side of the extractor. That is because the stress on these two lugs is slightly higher that the rest due to the altered geometry from the extractor slot.

Armalite remedied that, depreciating the lug opposite the extractor to allow the remaining 6 lugs to carry the load more evenly. I have about 17K rounds through my M15A2C (my first and highest round count AR), and the bolt is fine so far.

Armalite bolt on the left:

02.jpg

In the following image the red areas are area where the stress actually exceeds the yield limit for Carpenter 158 steel

I prefer 9310 bolts. Properly heat treated and tempered (more critical and more difficult than with C158), 9310 bolts are stronger. With modern manufacturing, this is much easier to accomplish. TBH, I'm not sure why anyone is still using C158 in 2016.
 
Armalite remedied that, depreciating the lug opposite the extractor to allow the remaining 6 lugs to carry the load more evenly. I have about 17K rounds through my M15A2C (my first and highest round count AR), and the bolt is fine so far.

Armalite bolt on the left:

02.jpg



I prefer 9310 bolts. Properly heat treated and tempered (more critical and more difficult than with C158), 9310 bolts are stronger. With modern manufacturing, this is much easier to accomplish. TBH, I'm not sure why anyone is still using C158 in 2016.
While effectively removing the asymmetrical lug evens out the loading, it also increases the load on the remaining six lugs by 17%, that 1000 pounds per lug goes to 1170 pounds. I haven't seen a stress model of an Armalite bolt, but I am pretty sure there are still a few areas where the stress still exceeds the yield limit. A better solution to this problem is to increase the radius at the back base of the lug (the red area). One manufacturer does this (see image below).

Why are there still Carpenter 158 bolts? Because that is still what's on the drawing. And while 9310 is slightly stronger, it has different fatigue properties, so bolt life is pretty much a wash between the two materials...

LMTBolt.jpg
 
eliminate a lug, or dont, theres enough meat on the lugs themselves to handle a lot more than they do, its an insanely flawed bolt design, the strongest bolts stick with two or three lugs.. check out the bolt on the leader T2 rifle, thats one of the simplest to produce, strongest bolts out there, its also similar to what the barrett 50 cals use
 
"Why are there still Carpenter 158 bolts? Because that is still what's on the drawing. And while 9310 is slightly stronger, it has different fatigue properties, so bolt life is pretty much a wash between the two materials..."
Meanwhile, the AK and a lot of other overbuilt designs (below the fatigue limit is what I mean) don't really care nearly as much about microradius stress risers or high tech tempering schedules.

The AR truly is an aerospace design, complete with a lot of engineering consideration that is quite simply not needed on more overbuilt arms. Not a pro, since it makes the design less flexible in the end & ultimately weaker in the face of overload; not a con, since it allows this gun to do the same job with less & typically accounts for planned servicing. It's just a design philosophy that isn't usually seen in the arms industry ("full-steel, heavy-iron, rah-rah-rah!") since the buyers are usually too big of cheapskates & skeptical to throw money at high level R&D up front. It took fifty years, but they finally figured out how the make the AR as cheap as the AK, and only took about five-ten to make the weapon itself function competitively. No other platform or product in history has been given this kind of consideration to my knowledge, but it appears to be yielding enormous dividends at the end of the day.

"eliminate a lug, or dont, theres enough meat on the lugs themselves to handle a lot more than they do, its an insanely flawed bolt design, the strongest bolts stick with two or three lugs"
There's usually a reason for losing "a" lug, and if it's fatigue (which it is) then increasing its load even farther beyond the fatigue limit only serves to shorten life. As far as ultimate shear load for single hits, yeah they're pretty beefy; which is why those little notches can be cut in the bolt lugs pictured above (early-early lug designs like the Hotchkiss Portative based on interrupted threads actually had triangular profiles like that bolt, called 'buttress threads').

As far as the stress 'analysis' linked above; that mesh looks awful coarse for the size of the critical areas showing up; could just as easily be an artifact of the model, or just a calculation quirk. Even still, FEM is too often flawed in the modelling constraints back-end with bad/unrealistic assumptions, so a single pretty picture is really nothing to go on (especially for a dynamic load case like we have, that's just plain hard to model accurately). All that said, I saw a similar analysis done for a custom "510 Reedwhacker" AR15 bolt whose bolt face was opened past the lug notches to accommodate the huge 300WSM base cartridge, and at 45-70 bolt thrust levels that model was not showing critical, and the rifle eventually built has seen plenty of use ('plenty' being a much lower number for a cartridge similar to 50-90, vs the original 223, of course) and continues to function so far as I know.

The fundamental problem with getting anywhere close to these material limits is that they are themselves statistically derived. That means that your Carpenter 158 bolt is not your buddy's Carpenter 158 bolt, and either/both could fail sooner --or later-- than analytically predicted. The official properties cover a large swath of the bell curve, but there's still that chance your part falls outside it. That's not even accounting for the accountable stuff like machining/smelting imperfections, or the speck of dust that causes 'headspace' to change imperceptibly on a lug with every shot, altering the load it experiences. Heck, alpha particles probably figure in if you stare at the problem hard enough :p. If the gun is built even a bit beefier, though, it all becomes a complete non-issue.

Think about how often bolt lugs would fail if you ran 223 on an AR10 bolt/extension, under any set of operating conditions ;)

TCB
 
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