The U.S. Army has been under pressure for years to use non-lead bullets. That's because training and combat use of army 5.56mm weapons puts 2,000 tons of lead back into the environment each year. This lead was originally taken out of the environment to be temporarily stored in the form of bullets.
While this non-lead policy burnishes the army's image and environmental cred, it also provides troops with an inferior bullet; the M855A1 copper alloy slug. But inferior to what? Well to a another new bullet. SOCOM (Special Operations Command) has developed a new 5.56mm bullet, the SOST (Special Operations Science and Technology) round. The SOST bullet solves a problem the M855 has long had, the inability to penetrate things like automobile windshields. SOST uses lead, and also has more killing power than the M855 (that did not inflict as much internal damage, and bleeding, as 7.62mm and 9mm rounds.) The M855A1 didn't solve these problems, but it was "green" (less polluting).
The U.S. Marine Corps was going to switch to the M855A1, but changed their minds when they found out about the SOST round. Army troops would like a heavier M855A1 bullet, not really caring if its made of lead or copper. Soldiers would like the SOST round, but the only army personnel getting that are Special Forces troops. The army has spent over $32 million developing the M855A1. SOCOM spent a lot less developing SOST, which has a bullet that weighs as much as the M855A1 slug, but is based on a popular hunting bullet design (the Federal Trophy Bonded Bear Claw). SOST is more accurate than the M855A1, as well as being deadlier and having greater penetrating power.
The army is now working on an environmentally correct 7.62mm round, and ignoring troop requests for the SOST round.