Throat erosion has nothing to do with reliability.
No; but gas port erosion will effect reliability and is cause by pretty much the same things that cause throat erosion. I mentioned throat erosion just because it was the first and most obvious example of the difference between 3,000 rounds in say three days vs. one year.
The test showed that the chamber and bolt face of the M4 barely got above 115˚F during a rapid fire session even when the gas tube melted after 264 rounds. This isn't hot enough to have any appreciable effect on spring k value.
I guess it depends on what the spring is made out of; but there isn't much question that some springs fail at a very low round count. Perhaps it is just bad building that kills them rather than heat.
As for extraction of the brass, I'd like to see some data on fired case temperature as a function of rate of fire and then see that data correlated to extraction force required. The POF didn't have any extraction issues during the 1,036 rounds fired at a rate of fire that no one in their right mind would try to emulate. The bolt face was just over 100˚F so how would this stress the extractor spring?
The difference between lock time in a rifle and a carbine length gas system is 175 microseconds. 175 millionths of a second; yet that difference is enough to cause the introduction of the enhanced extractor spring in the M4 carbine. The enhanced extractor spring is necessary because with the shorter time, the brass has not had as much time to shrink away from the chamber before extraction begins and the extractor spring that worked fine in the rifle was not always doing the job in the heavier use encountered by SOCOM.
As the chamber heats up, brass takes longer to obturate. At some point, the case will overcome the ability of the bolt to extract it and the extractor will either slip off the rim or bend/bite the rim. That may not stress the extractor or extractor spring all that much; but it will cause a stoppage that you'd have never seen if you only shot the rifle 3,000 rounds a year.
Which is really the major point I was trying to make, a lot of people never see problems with their rifles because they don't shoot their rifles to anywhere near the point where they would fail.
So I don't buy into the notion that taking a class and firing 1,000 rounds in a day is significantly different to firing 1,000 rounds in a month. If it works, it works!!
Let's say you've got your Sendra-PWA rifle and it is great. It has never failed you in 3,000 rounds and is deadly accurate. Of course that fastest you have ever fired it is 10 shots in 60 seconds. You take it to a training course and discover that after 100 rounds in a few minutes, it has stoppages. Is it because the gas port was drilled out to accomodate low-powered commercial .223 ammo and now it is overgassed? Is it because the chamber is much tighter than the "5.56 NATO" stamped on the barrel in order to help produce that stellar accuracy? Is it because the buffer spring was never right to begin with and so the rifle is cycling much faster than it is supposed to? Some combination of these or any of a dozen other possible issues? The heat didn't "degrade" the rifle - it just revealed where compromises in the original design had been made in order to offer some other advantage (enhanced accuracy, reduced cost, etc.) If you never fired the rifle 1,000 rounds in a day, you'd probably go on thinking that your rifle was flawless.
All I can tell you is in three of the four formal carbine training courses I've taken, there was at least one rifle that did fine as we sighted in and started out hunky-dory; but then as the day progressed and the rifle got hot, it stopped working. Every one of those guys had never had a problem with the rifle prior to the course; but there we were trying to fix it in the middle of the course. So I've got to disagree with your "If it works, it works" theory.
Machines are built to operate reliably in a certain range of conditions, if you exceed that range, they don't work so well. The great thing about rifles that are built to the TDP is not that they are the best rifles or have the widest range of operating conditions, it is that thanks to Uncle Sam and 50 years of research, we have a pretty good outline of what those conditions are.
