This Stag Arms bolt carrier assembly is designed to work with any Stag 15 rifle, upper half, or barrel designed for left hand ejection in the either 5.56, .223, or 300 BLK. The salt bath nitride process (also known as QPQ) creates a much harder surface that is, easy to clean, will run with less than ideal lubrication and resists wear for a lasting corrosion resistant durable finish.