500 rounds Federal Lake City T556TNB1 - 5.56mm Enhanced MK318 Mod 0 62 Grain SOST OTM - U.S. Military Barrier Blind and SBR Round

A short-barreled SCAR-L equips one of America's finest, somewhere in Afghanistan

The United States Special Operations Command is using a new 5.56 NATO cartridge, and now the Marine Corps is trying it out as well. Some folks refer to it as the SOST round, others call it the OTMRP round, the phrase “barrier blind ammo” has been tossed around the internet, and the official Navy designation is Mk 318 Mod 0. No matter what name you use, it seems that everyone except the US Army wants to load their rifles with it.

In response to the 9/11 attacks, our country went to war in Afghanistan in the fall of 2001. It didn’t take long for the troops to complain that the 1980s era 62 grain M855 ammo used in their M4A1 rifles was ineffective. In 2002 a big report detailing these problems was written up by the Naval Surface Warfare Center in Crane, Indiana and sent to the Pentagon. In 2003, America opened a second front in Iraq, and more information began coming in. The new war stories, combined with additional scientific testing, began to weigh on the Pentagon, and in 2005 they issued a formal request to the ammunition industry for “enhanced” ammunition. Intimidated by the complicated military procurement process, nearly every ammo maker in the country turned away. The Federal Cartridge Company was the only business to respond to the government’s request.

The Navwar/Crane and Federal/ATK bunch worked together quickly. This “Special Operations Science and Technology” team knew what they wanted and how to get it. Performance objectives for the new ammo were as follows:

• Increased consistency from shot to shot, and from one lot of ammo to another, regardless of temperature changes.
• Accuracy in an M4A1 rifle always better than 2 minute of angle (2 inches at 100 yards, 3.9 inches at 300 yards).
• Increase stopping power after passing through “intermediate barriers” like walls and car windshields.
• Increased performance out of short-barreled carbines such as the FN SCAR, while at the same time decreasing muzzle flash.
• Keep the cost as close to the old M855 as possible.

It was a tall order, but the first prototype batch of ammo was delivered to the government in August 2007. Increased velocity and decreased muzzle flash were accomplished by tweaking the type of powder used, but the real magic was found in the bullet design. The bullet was named the Open Tip Match Rear Penetrator. The front of it is a hollow point backed up by a lead core, but the lead core only goes about halfway down the length of the bullet; the rear half is solid brass. When the OTMRP bullet hits a hard barrier, such as the windscreen of a car being driven by a suicide bomber, the front half of the bullet smooshes (that’s a technical term) against the barrier, breaking it so the “penetrator” half of the bullet can fly through and hit the target beyond. This “barrier blind” bullet acts like two bullets in one, the second brass bullet flying exactly through the hole made by the first lead bullet.

This cutaway drawing shows the "reverse drawn" open-tip design of the OTMRP bullet. Photo by Federal Cartridge

Special Forces often use modern hollowpoint ammunition forbidden to the rest of the military. They do this by classifying themselves on paper as “counter-terrorist” forces which can follow law enforcement guidelines rather than military law. To be fielded by an entire branch of the military, the new round could not be classified as a hollowpoint by the Pentagon. Federal Cartridge helpfully pointed out to Pentagon lawyers that the SOST bullet uses a new “reverse drawn” forming process. The base of the bullet is made first, the lead core is placed on top of it, and then the jacketing is pulled up around the lead core from bottom to top. They said the bullet isn’t a hollowpoint, it’s an “open tip”, and the reason why the tip is open is just a byproduct of the manufacturing process, and has nothing to do with the terminal ballistics of the bullet’s stopping power in soft tissue. The lawyers bought the explanation, and with a wink wink here and a nudge nudge there, officially classified the new round as “Mk 318 Mod 0″, legal for the military to use according to the laws of warfare. In completely unrelated news *cough*, the round is said to be devastating against bad guys. The front half of the bullet fragments very consistently, creating what has been described as a “snowstorm” of lead in the first few inches of soft tissue. The solid copper rear of the bullet then penetrates around 18” of ballistic gelatin while tumbling. Ouch. The SOST bullets peform this way even with the reduced velocity of a 10.5″ chopped barrel. No wonder the Marines decided to buy “a couple million” rounds of the ammo to try out as part of a 10.4 million round ammo purchase in September 2010.

The only branch of the military not to show any interest in the new round at all is the US Army, which is instead deploying its new M855A1 “Enhanced Performance Round,” also known as the lead-free or “environmentally friendly” round. The Marines also bought 1.8 million rounds of this ammo as part of the same September 2010 order mentioned above. The M855A1 is a solid copper bullet topped with a 19 grain “stacked cone” alloy steel penetrator tip. The Army touts the fact that the M855A1 can penetrate 3/8-inch thick steel at 400 meters and also has “barrier blind” properties. Some observers say that the Army is dead set on buying ammo from the development program it paid for, and won’t buy ammo developed by the Navy no matter how good it might be. Others say that with budget cuts coming soon, the Army is anxious to advertise itself to influential Congress members as the most environmentally friendly branch of the armed forces. Perhaps the Army’s testing has convinced them that M855A1 really is a better round—all we know for now is that they aren’t interested in Mk318 Mod 0.

Interested in trying the SOST round? You can! BVAC makes a round which they advertise as being “Made in the USA to the same specifications as Mk 318 Mod 0”, and Federal has released a civilian version as well under the not-catchy-at-all name AB49. Because an executive order by President Bill Clinton banned the sale of “surplus” American made military ammo, Federal advertises AB49 as “loaded similar to Mk 318 Mod 0.” But lets not kid ourselves the way the government does. In all likelihood there is only one assembly line producing this ammunition for Federal Cartridge. When the assembly line is finished making its allotment of ammo for the government’s order each week, it runs for awhile longer making some extra for public sale. The official government NSN number for the ammo is “FC-10C801-013.” That number is stamped on each cardboard box of Federal AB49. Hint, hint, civilians.