Q. So why don't all US military units carry M855?
The original ammo for the M16 was M193, with a 55gr copper-jacketed lead-core bullet. The rifling twist on the first M16s was 1 turn in 14 inches, or 1:14. This twist rate was selected simply because it was the twist rate commonly used in the .222 Remington-chambered varmint rifles that the .223 round was based on. During tests of the M16 in arctic regions, it was found that the slow 1:14 twist wasn't fast enough to stabilize the 55gr bullet in the denser air. To correct this problem, the twist was tightened to 1:12 and all future M16s and M16A1s came with 1:12 barrels.
The M855 round and particularly the M856 tracer round, are very long bullets and require a faster twist rate in order to be stabilized in air. Firing M855 from a 1:12-twist rifle would result in an understabilized bullet that would only fly straight for about 90 yards, then veer off as much as 30° in a random direction. In order to prevent soldiers from accidentally firing M855 in 1:12-twist rifles, M855/SS-109 was given a green-painted bullet tip. This allows M855/SS-109 to be differentiated from plain-tipped M193. M16A2s, A3s, A4s, M4s and M4A1s all have a 1:7 twist and can stabilize both M855 and M193.
Fact: Stabilization is a factor of caliber (bullet diameter), velocity, and bullet length, not bullet weight.