An unserviceable firearm is defined as one which is incapable of discharging a shot by means of an explosive and which is incapable of being readily restored to a firing condition.
An acceptable method of rendering most firearms unserviceable is to fusion weld the chamber closed and fusion weld the barrel solidly to the frame. Certain unusual firearms require other methods to render the firearms unserviceable.
An unserviceable NFA firearm is still subject to the controls of the NFA, but may be transferred tax free as a curio or ornament.
[26 U.S.C. 5845(h) and 5852, 27 CFR 479.11and 479.91]