The Marine Times reports that the USMC have adopted the new SOST (Special Operations Science and Technology) 5.56mm ammunition …

The open-tipped rounds until now have been available only to Special Operations Command troops. The first 200,000 5.56mm Special Operations Science and Technology rounds are already downrange with Marine Expeditionary Brigade-Afghanistan, said Brig. Gen. Michael Brogan, commander of Marine Corps Systems Command. Commonly known as “SOST” rounds, they were legally cleared for Marine use by the Pentagon in late January, according to Navy Department documents obtained by Marine Corps Times. SOCom developed the new rounds for use with the Special Operations Force Combat Assault Rifle, or SCAR, which needed a more accurate bullet because its short barrel, at 13.8 inches, is less than an inch shorter than the M4 carbine’s. Using an open-tip match round design common with some sniper ammunition, SOST rounds are designed to be “barrier blind,” meaning they stay on target better than existing M855 rounds after penetrating windshields, car doors and other objects.

The full name and designation of the round is the MK 318 MOD 0 “Cartridge, Caliber 5.56mm Ball, Carbine, Barrier”. The 62 grain bullet was designed by Federal / ATK. It features an open tip with lead at the front and a thick copper base. The lead is designed to defeat barriers and the copper to penetrate the barrier.

Trophy Bonded Bear Claw

Many have said that it closely resembles the Federal Trophy Bonded Bearclaw bullet. Personally I think the similarities are superficial.

It should be noted that the bullet is open tip, not hollow point. Bartholomew Roberts explains …

It isn’t a hollow point. It is an Open-Tip Match round much like the M118LR. The jacket is drawn from the base (instead of the cheaper method of jacket drawn from the nose and an exposed lead base) to the tip of the bullet. The tiny little hole there is just a remnant from jacketing the bullet that way. It isn’t designed for expansion or calculated to cause unnecessary suffering, so it doesn’t violate the Hague conventions

The load has been optimized for ballistic performance and reduced muzzle flash from short barrels. From a 14″ barrel is achieves 2925 fps.

A 7.62mm version of this round, the MK 319 MOD 0, has also been developed. It also has been optimized for short barrels (16″) but also for reduced recoil. I have not heard of any adoption outside the special forces.

[ Many thanks to Matt Groom and Stu C. for the info. ]

UPDATE: The Marines are not dumping the standard M855 round, but will be the SOST alongside it in situations where the SOST will be more effective.