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
"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