Drop a line to firstname.lastname@example.org
If the barrel comes out of the P90, then they can cut it down, crown it, thread it, and weld a suppressor on the gun, if that's really what you want to do.
However, it's worth having the two stamps. Best kind of stamp collecting, if you ask me.
Also look into a "one suppressor fit's most" kind of setup. The Liberty Mystic, for example, is a 9mm can that is user serviceable, and can be used with .22, 9mm, .223 for limited use. You can find it on THIS PAGE
They don't list the 5.7 specifically, but I think if you talk with them, you'll find it will do fine on the 5.7.