The single parameter that I am aware of is that they have to fire NATO rounds. But other than that, if the Marine/Soldier/Sailor/Airman can get their hands on them, it is pretty much no holds barred.
And the whole "NATO rounds" thing is presuming that anyone is paying attention. When I went to Desert Storm, I took about 200 rounds of 9mm hydro shock ammunition with me (a gift from a buddy who had just gotten out of the Marines).
But we had a shakedown en-route and I had to chuck them in the drink. Getting caught with them in my personal gear on the ship was a Court Martial offense.
Special forces have a way of...shall we say...coming in under the radar...on such matters.