An examination of how it got that way is in order. M855 ammo fired from the barrel of the M16A2 rifle was effective on unprotected personnel to 500 meters.
In my experience, this isn't 100% correct. M855 has been found to be wanting in terminal performance since it starting being used on bad guys, not to mention the inconsistency lot-to-lot. This was first evident in '93 in somalia. SOF users preferred to use the older 55 grain A1 round there for operational use because it performed better in those 90 pound savages than green tip. The current "go-to" as of late in the SOF community is the MK262 (77 grain) or (if you are lucky enough to obtain them) the 75 grain Hornady BTHP match. We used both, and enjoyed the improvements in accuracy and terminal performance. I have heard great things about the newer 69 grain Barnes, but I never had the opportunity to shoot anything but paper with it.
Then the US Army:
1. Sawed off the M16 barrel to 14.5 inches and called the gun the M4/M4A1. Result: Muzzle velocity was reduced.
Yes. Designed for and adopted by SOF forces, then later the rest of the military. Prior to the M4, I was issued a MK727 carbine As the soldier's load increased and the missions changed from the cold war era, I never heard any of the conventional guys bemoaning the loss of their A2's.
2. Sawed the barrel off again to 11.5 inches. In some cases the barrel was shortened to 10.3 inches. Result: Muzzle velocity was further reduced.
These weapons (MK18 and similar) exist almost exclusively in the SOF community. They are also used by USN personnel who search vessels at sea. They are primarily used by SOF and the navy search guys in order to keep the overall length of the weapon down to a manageable size for close range engagements- especially when a suppressor is attached. This is particularly handy when engaging in close combat, not to mention fast roping from a hovering helo, while in free fall, swimming, etc.- with your 100 lbs of lightweight gear. When most of us were doing missions that didn't involve such activities, we mostly carried the M4 with the standard 14.5" upper, and some type of magnified sight. Or a MG or sniper rifle.
3. Developed a new round, the M855A1 Enhanced Performance Round; the so called "green round", with a hardened steel penetrator and a copper slug. That bullet will penetrate 3/8 inch of mild streel plate at 300 meters.
IF you can hit the plate at 300 meters. While the accuracy is slightly better than the old green tip, it is nowhere near the level of the other rounds I mentioned, and is still wanting in terminal performance.
4. The M855 round generates about 52,000 psi when fired. The M855A1 round generates about 62,000 psi. That increase causes barrels to wear out faster; sometimes bolts break and bolt lugs shear. Years ago i obtained 200 rounds of M855A1 ammo. A friend loaded up ten rounds in his old Colt AR-15. The sixth round broke the bolt.
Yep, it absolutely causes barrels to wear out faster- and the projectiles have also been found to damage feed ramps. As a civilian contractor overseas, we got most of our munitions from military stocks, but our weapons were not standard US military issue. We were specifically directed NOT to fire the M855A2 through our weapons due to the accelerated wear, and the fact that our guns weren't 100% compatible with military guns for repair purposes. The only catastrophic bolt failures I saw in my 23 years in the mil were from guns that had seen heavy use in full auto. The bolts would actually lose their temper and fail- and not always while the full auto jackassery is happening. As for me, I have an AR (no full auto) on which I have shot out 3 barrels in 18 or so years. I have also broken many other parts on this rifle over the years. I have fired 2-3000 M855A2 rounds from it. It still has the original bolt.
Between the numerous issues that surround the M855A1 round, and the fact that there much better rounds available that have been combat-proven, the idea that such a turd of a round was adopted by the military makes me want to puke.
5. The designer of the AR-15/M16 rifle is L. James Sullivan: He scaled down the AR-10 rifle designed by Stoner. Mr. Sullivan has recently re-designed some parts of the M4/M4A1 rifles in order to make them more durable.
i'm a retired US Army M/Sgt. After the problems associated with the adoption of the M16 rifle, i'm not sure the Army will do the right thing.