Check out the new 300 AAC Blackout from Advanced Armament Corp.
Introducing the Advanced Armament Corp. 300 AAC BLACKOUT (300BLK). This system was developed to launch 30 caliber projectiles from the AR platform without a reduction in magazine capacity and compatible with the standard bolt.
Full power 123 grain ammunition matches the ballistics of the 7.62x39mm AK, has 37% more energy than 5.56mm M855, and 9% more than 6.8 SPC TAP 110. In fact, from a 9 inch barrel, the 300BLK has more muzzle energy than 5.56mm M855 from a 16 inch barrel. When 300 BLK is used in a 16 inch barrel, it has 23% more energy than 5.56mm M855 from a 16 inch barrel - with much higher-mass projectiles for a more dramatic effect on the target. Or choose subsonic cartridges for optimal use with a sound suppressor - 220 grain Sierra OTM (open-tip match) bullets vastly outperforms a 9mm MP5-SD in penetration and long range accuracy.
Due to the high efficiency of the cartridge, less powder is used than 5.56mm, which results in a rifle that is a comfortable to shoot - even with a short barrel.
Reloading dies available from Forster products, Reamers and headspace guages available from Pacific Tool and Gauge.