I realize this thread hasn't been active for a while, but I wanted to ask, is it not the case that "once a machine gun, always a machinegun" not only applies to machine gun receivers, but any gun that gets registered as a machine gun, whether it's in working order or not(?). More broadly put "once a registered NFA item, always a NFA item" (?) since I can't take my suppressor apart and sell the tube as a non-nfa part. What do you have to do to make a NFA item, no longer a NFA item? I've got Sionics suppresors that cost more to sell and transfer than they are worth. What is the process for deactivating them so that the NFA will consider them deactivated? Do I need to get a torch and have an ATF agent witness me cutting them up? Can I send them to ATF for destruction? How do I get something in writing to show that the suppressors are destroyed?
Also, as for the M2 Carbine discussion which this thread originally was mean't to address, yes, my experience is that there are a number of tranferable M1 conversions that are set up for select fire, some good, some crap. They are not M2 Carbines just because they are set up like a M2, anymore than an AR-15 conversion becomes a M-16 because it's select fire. To be an M2 Carbine, wouldn't it have to be either a M1 overstamp, or Factory stamped as a M2? Plainfield made M2 Carbines, but I don't know much about them.
The $5k to $6k figure may apply for one of the many M1 Carbine conversions, but there are C&R Transferable Military M2 Carbines that are a good deal more valuable, especially to collectors.