ATF Definition: Reactivation & Unserviceable Firearm
Reactivation. The restoration of a registered unserviceable NFA firearm to a functional condition. This action incurs the making tax liability.
Unserviceable Firearm. One which is incapable of discharging a shot by means of an explosive and incapable of being readily restored to 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.