You can have the lower machined to accept your sear and actually have a spare FNC in which you could quickly install your sear from the original gun if you need to. The sear itself is considered the machine gun and is the registered gun. It is not assigned to any gun and is transferable to any other FN FNC. There is no law requiring you to register a gun that has been modified to accept a sear. However, it is illegal for you to own an unregistered sear if you own a gun capable of being converted to full auto with that sear. Being as the upper on an FNC is the actual part of the gun that is considered the registered part of the gun you could actually just have a spare upper with a GB barrel installed and if ever needed just switch the upper off your sear gun and install the new upper. You would not need to have the original lower modified to accept the sear.
