Complete M16-profile bolt carrier group for AR-15 / M4 rifles in 5.56 NATO (.223 Remington) and 300 Blackout (7.62x35mm). Full-auto carrier profile gives the carrier more mass behind the bolt for a more reliable cycling impulse vs. semi-auto profiles - fine for semi-auto rifles, preferred by most builders for duty / range rifles. Fit: AR-15 / M4 upper receivers chambered in 5.56 NATO or 300 Blackout. ~11 oz assembled weight. Carrier: Machined from 8620 steel Heavy-phosphate coated Chrome-lined interior Properly staked gas key with grade-8 screws Bolt: 9310 steel bolt - MIL-SPEC heat-treat and shot-peening Magnetic Particle Inspected (MPI) Chrome-lined gas rings Drop-in for any standard AR-15 upper. No tuning required for standard gas systems. Pair with a quality charging handle and you have the foundation of a reliable build.