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The M855A1 features a steel penetrator on top of a solid copper slug, making it is more dependable than the current M855, Army officials have maintained. It delivers consistent performance at all distances and performed better than the current-issue 7.62mm round against hardened steel targets in testing. It penetrated 3/8s-inch-thick steel at ranges approaching 400 meters, tripling the performance of the M855, Army officials said.
The Corps had planned to field the Army’s M855A1 until the program suffered a major setback in August 2009, when testing revealed that some of the bullets did not follow their trajectory or intended flight path.
The earlier design of the M855A1 featured a bismuth-tin slug which proved to be sensitive to heat, prompting Marine officials to stick with the M855 and also the Special Operations Science and Technology round developed by U.S. Special Operations Command instead.
Commonly known as SOST ammo, the bullet isn’t environmentally friendly, but it offered the Corps a more effective bullet, Marine officials have said.
I’m a Retired Army oggicer and I agree with th Corps.