Once upon a time, the U.S. Army and U.S. Marine Corps wore the same woodland camo pattern and carried the same M855 5.56 x 45mm ammunition in their magazines. If Congress gets its way, that will be the case once again—for the ammo, anyway—according to the Marine Corps Times.
Currently the Marine Corps uses a mix of the M318 SOST round and legacy M855 ammo while the Army uses the M855A1 round. As a cost-saving measure, the House Armed Services’ Subcommittee on Tactical Air and Land Forces has ordered a study of potential uniform 5.56 ammunition, prompting a showdown between the two rounds.
Both rounds are ballistically similar, delivering sub-two-minute-of-angle accuracy. However, the Army’s round has been found to tear up the workings of a weapon faster.
The Marines are moving to make the SOST their standard round and only use M855 rounds for training until stocks are used up.
The development of both rounds came at almost at the same time. The old M855 ammo has standard 62-grain lead core bullets with a steel penetrator meant to penetrate light body armor and steel helmets. When Marines began engaging enemies in Afghanistan, reports of M855 ammo failing to kill or wound combatants in battlefield situations came pouring in. The bullets couldn’t pass through barriers like windshields or plywood without losing a lot of their mass, rendering them ineffective on the other side. They just weren’t delivering the terminal ballistics required.
In 2005, the Army began using the new M855 “green tip” ammo, which replaced the lead core with tungsten to make an environmentally safer round. But things weren’t going so well in testing. The rounds had a tendency to yaw in flight and hit their targets going sideways.
So the Marines looked elsewhere for a new round and developed the M318 Mod 0 Special Operations Science and Technology round, or the SOST for short. It is a barrier-blind round, meaning it can pass through obstructions like sandbags or sheet metal and still inflict grave injuries on a target. The USMC first fielded the SOST in Afghanistan in 2010 and claimed to see an immediate benefit.
Meanwhile, the Army worked on its new ammo and came up with the final M855A1 round, which contains no lead. A new production method led to a cleaner boat tail, eliminating the yaw problem. The steel alloy core remained. The rounds were found to fail at high temperatures when tested in 2009, causing a one-year delay. The new round was finally fielded in 2010.
The Marines weren’t about to abandon the SOST, which performs well, for a round they saw as plagued with design problems. Though if it is determined that switching the USMC to the Army’s M855A1 ammo is indeed the most cost-effective option, the Marines will be forced to make the switch.
There are other problems with the Army’s round that gave The Corps pause. The exposed steel tip has a tendency to gouge the feeding ramp on a firearm after a number of rounds have been chambered. The round’s high chamber pressure can also crack bolts and erode barrels and has a tendency to cause early erosion in a weapon’s gas port–a big problem for the M4 carbine, which as a consequence, cycled faster, especially during full-auto fire.
To address these problems, the Army has moved to a single manufacturer for all M855A1 ammo and designed new magazines to prevent damage to feed ramps.
You can read more about other costs that would be associated with the move and differences between the rounds here on Marine Corps Times.