I bought a MAC-10 .45 sub-machine gun years ago. Fill out the paperwork, get pictures, prints, CLEO sign off and $400 for the tax stamps, (one each for gun and suppressor). Took about 3 months to get the paperwork back from ATF. Picked up the gun...had fun.
It was a straightforward and simple process.
Only problem was that I needed an ammo factory's worth of ammunition to keep the 1200 RPM beast happy.