Note that the silencer definition applies only to devices for firearms, i.e. powered by an “explosive”. An air gun silencer is not covered. But if it can be used on a firearm it would be. Thus an airgun silencer permanently attached to the airgun, or too flimsy to be used on a firearm, should be exempt. If you have an interest in pursuing this line of thought submit a sample or drawings to ATF Tech. Branch. I am not aware of any airgun silencer currently made, or determined to be exempt from this definition. But clearly there is room under the definition for such a gadget. Likewise, since antique guns, as defined in the GCA are not “firearms”, a silencer for such a gun is not, or should not be, covered. Perhaps one fitted permanently to a pre-1899 gun? The mind reels.
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A DEWAT machine gun transfers tax free, as a “curio or ornament”, on a Form 5. To be a DEWAT, a gun should have a steel weld in the chamber, and have the plugged barrel steel welded to the frame or receiver. Having said that, a gun may be registered as unserviceable and not be de-activated in this manner. It may have cement or lead in the barrel, or a piece of rod welded, soldered or brazed in the barrel. Despite the repeated warnings from ATT, apparently DEWATs were made or imported that did not have steel welds. And a weapon registered as “unserviceable” before 1968 was not held to these standards. One (ostensible) reason machine gun receivers were redefined as machine guns in 1968, thus bringing DEWATs under the NFA regulation, was that folks were regularly and easily making their DEWATs live guns w/o complying with the law. Some barrel plugs were so poor they would fall out with little coaxing. The thing with buying a DEWAT is that it may be easy to make it live, or it may be hard. The gun may be pristine or rather beat up. They usually cost less than a live gun because they will not be 100% original if made live. However if you just want a shooter buying a DEWAT and getting it made live can often be cheaper than an original gun. DEWAT guns are best not bought Rifle Scopes unseen, unless you do not wish to make it live, but have it as a wall hanger. The exact state and extent of the welds will determine how hard it is to make live. However if you want a wall hanger, a dummy gun is much cheaper, and requires no paperwork. They can look totally authentic. They do not have an intact machine gun receiver, but a partially machined dummy receiver.