Licensing requirements under the GCA for any person who installs “drop in” replacement parts in or on existing, fully assembled firearms. A “drop in” replacement part is one that can be installed in or on an existing, fully assembled firearm without drilling, cutting, or machining. A replacement part, whether factory original or otherwise, has the same design, function, substantially the same dimensions, and does not otherwise affect the manner in which the weapon expels a projectile by the action of an explosive.