Parts other than an actual weapon as the machine gun?  For example, when an SOT form 2's an HK sear pack as the machine gun, that can be used in a semi auto non machine gun.  What are there rules as to what can and cannot be registered?  

Thanks,

Brick

If the device allows a non-MG to fire automatically, it's registered as a machinegun, with some description about its "conversion device/apparatus" status in the additional description block.

If it doesn't have a barrel, barrel length is N/A.

If it doesn't have a barrel or an intrinsic caliber, caliber is N/A.

What I am looking to do is register the converted bolt carrier of an RPD as the conversion device.  It will be cut to fit the semi receiver.  The full auto trigger assembly will also be used, but I think I should not have to register that as you don't register the M-16 trigger group parts used when using a DIAS.

Brick

As long as the full auto trigger assembly fits the semi receiver without modifications, then the modified bolt carrier will be the conversion device, which you would mark and register. Pretty much the same thing as slotting a full auto bolt to fit a semi Uzi receiver.

If there are mods you have to make to other parts to fit the semi receiver, in order for it to function as an MG, then you might have to submit a sample to BATFE Tech Branch for determination as to which part they consider the conversion device, and which would be the part you'd have to mark and register.

How about with a Ruger 10/22 that has the hammer and bolt modified and a new sear made?

In a conversion like you described, most in the past have registered the sear, although a better choice would have been to register the trigger frame, which also probably required modifications, because it was the largest and most durable piece of the conversion parts set.

Think of an AR-15 DIAS - the body is the largest and most durable part, even though the sear is also different and a distinct, new part from a GI sear, so absent a BATFE directive, either could have wound up being the marked, registered part

M2 carbine conversion kits could register any one of the seven conversion parts, but most used the disconnector, which became the "machinegun", and the other parts could be replaced. Original M2-marked carbine receivers were considered RRs, and all the parts were just replaceable spare parts for M2 RR owners.

We registered the Bolt and the lower at my shop the lower has to be cut to fit the semi frame so we numbered both