Ever wonder why the Army and the Marines for so long used two different types of bullets for their M4s and M16s?

In 2010, the Army adopted a lead-free bullet known as the M855A1 for the M4 and M16A4 rifles.  From Military.com (bold italics added):

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.

However, in December 2017 the Marines appeared to give in to pressure from Congress.  A slightly earlier article from Military.com provides some background (bold italics added):

After repeated urgings from Congress to move to a common rifle round with the Army, Marine Corps officials told lawmakers Tuesday that they’re getting close to being able to do so.

The Marine Corps continues to use M855 ammo for their M16A4 and M45.56mm service rifles, instead of the M855A1 Enhanced Performance Round the Army uses for its rifles. . . .