Any "part or combination of parts" that can be used to make a semi-auto rifle fire full auto comes under Federal law as a machinegun, in and by itself, whether or not it is installed, and whether or not the owner also possesses a semi-auto rifle of the type it is designed to convert.
In other words, own an unregistered DIAS, get caught, go to jail.
Before someone flames me on carbine parts: no SINGLE part of the so-called "M2 Kit" can by itself convert an M1 carbine to M2 configuration, that is where the "combination of parts" comes in. The same rules apply. If you have the conversion kit, you have a machinegun. If it is registered, fine. Otherwise, it is illegal.
Jim