One of the main reasons U.S. Army dogfaces and U.S. Marine grunts don't fight together on the same battlefield is they use a different variant of the same type of 5.56 mm ammunition for their common Colt M4 carbines.

Army and Marine M4s both have a caliber of 5.56 mm and fire 5.56×45mm NATO rounds. The difference is in the fine print.

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Army M4s fire a variant of the 5.56×45mm NATO called the M855A1 Enhanced Performance Round (EPR). Marine M4s fire the standard M855 round for their M4s and M16A4 automatic rifles.

The army adopted the M855A1 EPR in 2010 to replace the M855, which was adopted back in the Cold War era.

The M855A1 features a steel penetrator encasing a solid copper bullet, making it is more dependable than the M855, claim Army officials. It also delivers consistent performance at all distances and performed better than the current-issue 7.62mm round against hardened steel targets in testing.

It penetrated three-eight inch-thick steel at 400 meters, tripling the performance of the M855.

On the other hand, the Marines planned to transition to the Army's M855A1 until the program suffered a major setback in August 2009. Despite this, the Marines continued testing the M855A1.

These new tests that began in 2015 seem to have convinced the Marines the M855A1 is the way to go -- with a lot of prodding from the U.S. Congress.

After persistent pleadings from Congress for over a decade, top Marine Corps officials said they're getting closer to transitioning to the M855A1.

"The good news with that round ... specifically the Army 855A1, is much better at penetrating armor. So that's a good reason to go with that," said Lt. Gen. Robert Walsh, Commander of Marine Corps Combat Development Command.

Gen. Walsh said a reason for this about face was that the Corps had sent the Army round to Helmand province, Afghanistan with a 300-Marine advisory element that deployed last April.

Ongoing testing at the Aberdeen Proving Ground in Maryland is examining performance; stopping power; effect on the durability of the weapons, and the impact of the flatter trajectory of the M855A1 round compared to the M855.

"We're working through reliability things in testing, but we will make some adjustments from that, and I think in the end our Marines will have a much better capability when we're done with it," said Gen. Walsh.

TagsU.S. Army, U.S Marines, 5.56×45mm NATO, M855A1 Enhanced Performance Round (EPR), M855