Don't forget you'll need a threaded barrel for that USP .40. You might be better off selling the USP .40 and getting either a USP-SD 9mm or USP .45 Tactical which would be ready to go for your silencer. Also a 9mm or .45 silencer would be a more versatile tool you can use on more weapons with arguably better suppression ability.
Don't worry about what gun you are going to use it on. You have 6 - 12 months to save your money for a pistol once you order the silencer. Good luck!