This gas operated semi-automatic packs a bigger punch than our 5.56mm/.223models – a .270 cal. 6.8mm SPC (Special Purpose Cartridge) specifically developed for the military, but now available commercially. Velocity averages a bit over 2600 FPS (with Hornady 115 Gr.V-Max), and the heavier bullet weights deliver excellent terminal energies. The 6.8 mm SPC was developed from a .30 Remington case which is shortened slightly to fit the AR15 type magazine. Due to the “fatter” case diameter, magazine capacity in a 30 Round AR type shell is reduced to 26 Rounds, and 6.8 specific springs and followers are used. The bolt is also different - due again to case diameter (take care not to interchange a 5.56mm/.223 bolt with a 6.8mm SPC bolt). - Busmaster Website

It's a R.H. Twist – 1 turn in 10”

Here are a selection of AR ammo types. The 5.56x45 at top and bottom, the .7.62x51 (.308) second from top, 6.8 SPR below that and 9mm below that

The Bushmaster 6.8mm SPC Rifle and an AR15 SBR in 5.56x45 (.223)