Both Uncle and Tam linked to what seems to me like an excellent article on the failure mode(s) of the M16/M4 system, which cemented, for me at least, a great deal of respect for the platform. If you haven’t read the whole thing, Do Read it.
It concludes (after much explanation of HOW the conclusions were derived);
“How to Deal With Heat Limits
The Training Answer: First, every GI should see those Colt test videos [of firing them to failure] and know what his gun can, and can’t, do. While the Black Hills guys were correct in noting that SF/SOF guys usually manually fire single shots or short bursts, even most of them don’t know what happens when a gun goes cyclic for minutes at a time. A good video explaining “why you can’t do that” would be a strong addition to training, not only for combat forces, but for support elements who may find themselves in combat and feel the urge to dump mags at cyclic rate.
The Morale Answer: Every GI should see the same done to AKs as well. There is a myth perpetuated by pig-ignorant people (like General Scales) that the AK series possesses magical properties and that the American weapons are crap. In fact, nobody I know of at the sharp end is at all eager to change, perhaps because the laws of physics and the properties of materials apply just as firmly to a gun originally created by a Communist in Izhevsk as they do to a concept crafted by capitalists in California. If you’ve ever fired an AK to destruction, you know that it grows too hot to hold, then the wooden furniture goes on fire, then, if you persist on firing it full-auto, it also goes kablooey. Not because there’s anything wrong with this rifle, but the laws and equations work the same for engineers worldwide.
The Systems Answer: As you can see from the Colt videos, if you clicked on over to Chivers’s article, thickening the barrel nearly doubled the rounds to catastrophic failure on cyclic. An open/closed bolt cycle might have practical benefits. They wouldn’t show up in sustained heavy firing like the destruction tests, but they might show up in how a weapon recoups from high temps, and open-bolt autofire would eliminate cook-offs, at least. But any such approach needs thorough testing.
The Wrong Answer: Replacing the M4 with something like the SCAR or the HK416, something that is, at best, barely better, that is much more maintenance intensive, and that, contra Scales’s assertion that his undisclosed client’s weapon is “the same price,” is twice (SCAR) or three times (416) the money. (The 416 mags are the best part of the system, though).”
I’ve fired over a thousand rounds in a day, both from an AK and a Ruger Mini-14, and didn’t come even close to failure, or even serious degradation of the rifles. (I haven’t tried it with the AR simply because my business hasn’t made products for it as yet) But then I’ve WORKED WITH steel since I was a kid, and know first hand how soft and moldable it becomes at high temperature, and I’ve seen how it can be “hammer welded” which is welding two pieces together that are red hot, just by hammering them together). And just about anyone who grew up on a farm understands “instinctively” that even the best steel becomes soft enough to bend like a pretzel, using nothing but hand and arm muscles, at high temperature, because THEY’VE DONE IT over and over. And so, without even having to think about it, it was natural for me to avoid over-heating the weapons. I’ve never had so much as a cook-off (again; as kids we sometimes cooked off naked rounds on purpose, because it was interesting and fun). I have no doubt than an AR-15 would do as well in a thousand round, one day test, though it may need a little attention to keep it cycling with the carbon that gets into the action. My Colt AR has been known to stop after about 350 rounds unless I keep it real wet (and depending on the particular ammo).
That one of the M4s in the battle related in the article was able to get through ~600 rounds in a very short time is pretty awesome. The physical limits of the steel were exceeded at that point, and physics is physics.
Another quote from the article stuck out to me as great. It addresses the trade-off between the ability of a weapon to fire an enormous number of rounds quickly without failure, verses the operator’s ability to actually carry it (because it’s too heavy). This may be a paraphrase, but it’s really close;
“You can carry it all day or you can fire it all day, but you can’t do both.” Yup. Take your pick.
But then if you’re unfortunate and pathetic enough to have gotten your technical and physics information from Hollywood actors you might believe that, “Fire doesn’t melt steel”.
I found it interesting to look at the “service life” of a typical rifle in terms of actual bullet-acceleration-in-bore time. If we assume a nice round number of one millisecond to push a bullet through the bore, and if we assume a nice round number of, say, 30,000 rounds to wear out the rifle, that’s a “service life” of thirty seconds. Compared to your family car’s engine, the combustion system in a rifle runs at awesome power levels, and with no oil pressure. Your actual mileage may vary.