An AR 15 Pistol is a ‘pistol’ variant of the AR-15 rifle platform, featuring a barrel shorter than 16 inches.
Unlike Short-Barreled Rifles (SBRs), AR Pistols are not subject to NFA regulation, though there are additional restrictions to remember when building a pistol. For example, AR 15 pistols cannot feature a rifle stock or vertical grip, but a ‘pistol brace’ may be used for additional support with the user’s dominant arm.
Otherwise, AR pistols are entirely compatible with standard AR-15 parts and components.