Check out the new 300 AAC Blackout from Advanced Armament Corp.
http://www.youtube.com/watch?v=HbYLyfFf9u8
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.