The 6.8 SPC delivers more energy than the 5.56mm NATO kinetic energy is anywhere from 40-50% greater at shorter ranges; up to 35% out to 600 yards. The 6.8 mm SPC is more controllable in rapid fire, and lighter, allowing operators to carry more ammunition than would otherwise be possible with the larger caliber round. The 6.8 SPC was designed for better terminal effectiveness at the shorter ranges of urban combat. 6.8 SPC II was designed to be a longer range cartridge; If all you're looking at is muzzle energy and long distance performance, then 6.8 SPC is the clear winner. It is a field tested and battle proven cartridge with a huge following that can put animals down at longer distances. The 6.8 SPC II upper receivers will fit AR15 lowers, but you will need to change the bolt though and use specific 6.8 mags. Hardened Arms LLC builds all 6.8 SPC II products to SAMMI Spec and highly recommends that you use only 6.8 SCP II SAMMI spec parts.