In general, even some components that make up an NFA item are considered regulated. For example, each baffle inside a silencer, if removed from the tube, is considered a silencer. Thus, individuals cannot legally own any part of a silencer without registering it with the BATFE and paying NFA taxes. Silencers and machine guns are the most heavily regulated. For example, in Ruling 81-4, BATFE declared that any AR-15 drop-in Auto-Sears made after November 1, 1981 is itself a machine gun, and is therefore subject to regulation. While this might seem to mean that pre-1981 sears are legal to possess without registration, BATFE closes this loophole in other publications, stating, "Regardless of the date of manufacture of a drop in auto sear, possession of such a sear and certain M-16 fire control parts is possession of a machinegun as defined by the NFA. Specifically, these parts are a combination of parts designed and intended for use in converting a weapon into a machinegun and are a machinegun as defined in the NFA."
Owning the parts needed to assemble other NFA weapons is generally prohibited. A person cannot own machine gun trigger components unless he owns a registered machine gun. The M2-Carbine trigger pack is such an example of a “combination of parts” that is a machinegun in and of its own. Most of these have been registered as they were pulled from surplus rifles in the early 1960’s.
Most current fully-automatic trigger groups will not fit their semi-automatic firearm look-alike counterparts – the semi-automatic version is specifically constructed to reject the fully-automatic trigger group by adding metal in critical places. This addition is required by the ATF to prevent easy conversion of le I weapons into machine guns. Additionally, the fully-automatic trigger group is also permanently modified in such a way that it cannot be made to function as a fully-automatic fire control device. The ATF has listed required manufacturing procedures for modifying these fully-automatic trigger groups to make them into legal semi-automatic trigger-groups for civilian sales.