For the quietest , it would have to be a single shot or a manual repeater to get rid of the action noise . I'd look at a T/C Contender or something similar .
For fun , though , the sky's the limit . I have a couple of Sparrows in ATF jail right now and I have a Colt /Walther .22 1911 with the proper nut , a S/W AR15-22 , and , now , a couple of nuts for the two Thompson FAs with .22 kits in them . I also have a spare barrel for a Colt Ace I'd like to thread and adapt as well , but can't find anyone to do it .
Chris