Tungsten ended up being just as toxic ( The round also had stability and terminal ballistics issues but that is another topic) as lead so they've replaced that one in the M855A1 with mild steel.
Then they had a fancy jacket made out of bismuth and tin alloy that appeared to be both not exactly environmentally friendly especially for the users due to buildup of bismuth salts firearm and unstable at high temperatures which causes inconsistencies in accuracy when fired from a hot barrel.
So they've pretty much moved back to a copper alloy half jacket and an arrowhead steel tip insert, the M855A1 also didn't solve all balsitis issues and ended up being slightly longer which can cause feeding issues.
Most of my barrels are ferritic nitrocarburized, so they should be impervious to anything... guessing that the US armed forces might have as fancy bore linings.
And don't be surprised if military guns are less decked out than yours they are built to costs.
Also remember that it's how often you clean your barrel and how many rounds you fire.
The M855A1 was designed to used in both assault rifles and LMGs so something like a SAW would fire 1000s of rounds before it can be properly cleaned up. Even an M4 would probably be much less cared for while on active duty than your match grade rifle.
Not sure they saved themselves a lot of money, copper is $. I knew they were trying a tin alloy at one point, not sure what happened to that other than Google saying "Unstable at high temperatures", which is interesting.