What a combo? Get started with your very own Left Handed build. This combo comes with a left handed bolt carrier assembly  , and an A3 Left Handed Flattop Upper receiver,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.