Many M4/AR operators use an 'O' ring over the extractor.
One reason the little 'O' ring is needed because the extractor spring goes south rather quickly and loses it's temper because of heat.
Hmmm, I hadn't heard the claim that the O-ring was needed because the extractor spring goes south due to heat and I am skeptical of that claim. If this were true, then we should see the same problem in the M16 series since it has the same gas system; but the problem never occured until the M4 was developed.
A much more likely explanation is that because the M4 tries to extract faster than the M16 while there is still a higher residual pressure in the chamber and the brass is still obturated, the original M16 extractor often slipped over the rim because the brass was still stuck to the chamber walls. The problem gets worse as the barrel gets hot - not because the spring has weakened its temper; but because it takes even longer for the brass to cool and shrink away from the chamber wall as the barrel gets hot (and with less mass the barrel heats up faster).
Note that you can fix this problem without using an O-ring at all just by going to an M4 extractor spring and black insert.
The tendency of the extractor and bolt lugs to be brittle.
I've never heard of an incident of a brittle extractor. There are plenty of reports of bolt lugs snapping off in M4 carbines; but this again is probably not related to heat. If heat were an issue, we would see the same thing in the M16.
I believe a more likely explanation would be the fact that internal pressures on the bolt are increased 1.5x in the carbine and some design issues on the lugs. For one, the lugs are undercut as a design compromise where they meet the extractor in order to have a wider extractor claw and distribute the force over more area (to prevent ripping rims off brass). Not a problem in the original design; but when you increase the force 1.5x - a problem. The second issue is the way the lugs are cut dramatically increases the stress where they meet the bolt. If they were just radiused there (like pretty much all of the enhanced bolt alternatives for the M4 did), it reduces the stress dramatically but requires new barrel extensions in many cases.
Finally, the M4 can continue to run with missing lugs on the bolt. The main danger that the broken lugs pose is not the actual absence of the lugs; but the tiny bits of broken lug floating around in the internall waiting to get jammed into the workings.
Tendency of the Military AR's to have an occasional Cook off post fire fight.
In the M4s, cook-off can happen after as few as 180 rounds fired back to back on full-auto. Cook off is a function of chamber temperature (about 600F). How would a gas piston weapon with the same characteristics have a lower chamber temperature than a direct gas weapon?
And of course overall durability. The military weapons don't last too long particularly with SPECOPS because of wear. Now those boys will wear any weapon out. They just wear the M4/AR systems faster than any of the others.
No question a gas piston would be more durable than an M4. Heck, you could make the M4 more durable DI just by moving the gas port forward two inches so that port pressure, internal bolt pressure, cyclic, dwell and all those other issues were more inline with the rifle. That is the nature of logistics - a change that makes perfect sense when you are only changing ten rifles starts to be downright backwards when you have to apply it to 1,000 rifles.
Janes said:
Nearly as bad, the hot gases and fouling directed into the AR's receiver cause the weapons to rapidly heat up under full automatic-fire conditions. This is not much of an issue with semi-automatic only guns, but with Special Forces carbines that are frequently fired extensively on full automatic, heating is critical.
No question that heat is bad for the internal parts of a rifle.... but this argument makes no sense to me. How does DI cause a weapon to heat up more rapidly than a regular gas piston? Even if we are just referring to the internal parts, the one critical part that is actually cited as the problem is the bolt - and the bolt is already in direct contact with the combustion gases in the chamber whether you use a DI or gas piston.