It's just the extra hole to accommodate the M16's full-auto sear. Anyone finishing an 80% AR15 lower could easily drill that third hole, but in the US the resul

It's just the extra hole to accommodate the M16's full-auto sear. Anyone finishing an 80% AR15 lower could easily drill that third hole, but in the US the resulting firearm would be considered "readily convertible" to fully-automatic and therefore only legal to manufacture or possess under a limited set of circumstances. Buying a bunch of M16 trigger parts, while legal in the US might attract unwanted attention, but I think a cartel with machine tools could easily manufacture these parts themselves, or a full-auto sear of a different design. If they're not making select-fire rifles, and it wouldn't surprise me if they're not, it's because using a fully-automatic rifle effectively requires a lot of training and discipline. Most people under most circumstances would use a rifle more effectively without having the option of full-auto; even the US military has questioned its value for the average soldier, using some models with 3-round burst instead.