[GoingQuiet]

A bare AR15 lower is neither a longarm or a pistol. It is considered "Other" on the 4473.

As it is not a longarm or a pistol - ATF has issued a directive noting such and that only longarms are subject to the age limits of being 18 to buy. Anything "other" than a handgun or a longarm is subject to a min age of 21.

This includes pistol grip shotguns.

That is where many of the "it is treated like a pistol" arguments can trace their roots as "if you have to be 21 to buy it, it must mean it is treated as a pistol" when it is not.