I can't comment on the legality, but I have shot an MP40. Two things stick out in my memory...
- There is a perceptible time lag between squeezing the trigger and the bolt/firing pin striking the cartridge. It's open bolt, and a fairly long travel...let me measure it...I get 3 5/8" travel [first shot]...I don't have anything to measure the time to ignition but it is a perceptible fraction of a second...maybe 1/4 sec, maybe 1/8...Whatever, it is disconcerting if you're used to firing a closed bolt SMG [M16]...
- The bolt is pretty heavy...When it starts moving forward, everything else moves back...First hits were rare...
And it's heavy...Especially fully loaded...My book says 10.4 pounds...(It also says an M1A1 Thompson is 10.75 pounds)...
That's about all I recall, these many years later...