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Hi Mark, 050806113930 is an AR15 BCG and 050806112278 is an M16 BCG. The M16 BCG can also be used in an AR15. M16 style configurations have more material toward the rear of the carrier, called a full auto lug. This additional material is necessary for the full auto sear to function. Due to the extra material, M16 bolt carrier groups have an increased weight. AR-15 style configurations have a cutout on the bottom of the carrier. The cutout on AR-15 carriers is the absence of a full auto lug and will prevent the BCG from being capable of being used for full auto firing. AR-15 bolt carriers weigh less than M16 carriers because less material is present, though typically not enough to notice in practical use. Stay safe & have a great day!