If you are making a NFA firearm on form 1 (like a short barreled rifle) you always pay the tax and wait till you get your stamp before you build it.
You can not legally assemble a new machinegun. Period. Even if you are using old imported parts.
Building a semi-auto is fine as long as the barrel is 16" or more and you have the 10 USA made 922r compliance parts. I think you get a pass around 922r if the gun is NFA...
Stopping power: The power some guns have to make you stop what you are doing and get your wallet out.