With regard to Military/LEO purchase of a Class 3 device... the rules and regs are no different for the individual officer as they are for us.
If the agency of unit was purchasing, different story. But if a cop/soldier/airman/whatever wants to buy, they've got to go through the Form 4, fingerprints, etc. The one cool thing about a soldier buying is that their CLEO is the post provost marshall. Chances are better than average that the PM is going to know what a machinegun is and might even say "Cool".