Interesting articles..........
U.S. Army Issues New M855A1 Ammo to Troops in Afghanistan
U.S. Army Issues New M855A1 Ammo to Troops in Afghanistan « Daily Bulletin
The military sought to replace current M855 ammo because M855 has not performed adequately in the Afghan theater. Specific complaints include: 1) inability to penetrate vehicle windshields; 2) poor long-range performance; and 3) failure to fragment even in short-range anti-personnel use. The Armys Picatinny Arsenal claims that the new M855A1 has improved hard target capability, more consistent performance at all distances, improved accuracy, reduced muzzle flash, and higher velocity. M855A1 ammo is tailored for use in M4 carbines but should also give enhanced performance in M16s and M249s.
USMC Adopts New Open-tip 'SOST' 5.56 Ammo
USMC Adopts New Open-tip 'SOST' 5.56 Ammo « Daily Bulletin
After learning that M855 NATO ammo does not perform well from short-barreled rifles such as the M4 carbine, the U.S. Marine Corps has started issuing a new type of 5.56×45 ammo to its troops in Afghanistan. The new SOST (Special Operations Science and Technology) ammo, officially designated MK 318 MOD 0 Cartridge, Caliber 5.56mm Ball, Carbine, Barrier, features a different open-tip 62mm bullet. The new bullet, with a lead core (in the top half) and solid copper bottom half, is similar to hunting bullets such as Federals Trophy Bonded Bear Claw. The SOST bullet was designed by Federal/ATK, which will produce the loaded ammunition.
U.S. Army Issues New M855A1 Ammo to Troops in Afghanistan
U.S. Army Issues New M855A1 Ammo to Troops in Afghanistan « Daily Bulletin
The military sought to replace current M855 ammo because M855 has not performed adequately in the Afghan theater. Specific complaints include: 1) inability to penetrate vehicle windshields; 2) poor long-range performance; and 3) failure to fragment even in short-range anti-personnel use. The Armys Picatinny Arsenal claims that the new M855A1 has improved hard target capability, more consistent performance at all distances, improved accuracy, reduced muzzle flash, and higher velocity. M855A1 ammo is tailored for use in M4 carbines but should also give enhanced performance in M16s and M249s.
USMC Adopts New Open-tip 'SOST' 5.56 Ammo
USMC Adopts New Open-tip 'SOST' 5.56 Ammo « Daily Bulletin
After learning that M855 NATO ammo does not perform well from short-barreled rifles such as the M4 carbine, the U.S. Marine Corps has started issuing a new type of 5.56×45 ammo to its troops in Afghanistan. The new SOST (Special Operations Science and Technology) ammo, officially designated MK 318 MOD 0 Cartridge, Caliber 5.56mm Ball, Carbine, Barrier, features a different open-tip 62mm bullet. The new bullet, with a lead core (in the top half) and solid copper bottom half, is similar to hunting bullets such as Federals Trophy Bonded Bear Claw. The SOST bullet was designed by Federal/ATK, which will produce the loaded ammunition.
I don't think that's going to chamber in the M4's 
