One of the gas blowback pistols you might want to have as part of your collection is the Western Arms M92FS Leon GBB. This comes with a compensator and a silencer making it a good hitman's weapon. With the Magna Blowback it will produce a hard kick. It's hard to find it with airsoft retailers with the exception of Japanese airsoft retailers.
Mach Sakai gives it a full review with the video version you can watch below (in Japanese):
