From SRT's website. Would highly recommend them. They converted a 22/45 for me years ago and it's been great.
Our Sound-Master Ruger MkII is available in several configurations. You have a choice of either stainless steel, or blued/molycoted finish, and with suppressor tube lengthsof , 6.2”, 6.5”, 7.1” (standard) and 7.8” ,as measured from the front of the receiver.
The longer the tube, the greater the suppression. The 7.8” tubes have a SPL (Sound Pressure Level) of about 112 db with high velocity ammo. Each increment of shorter tube length adds about 2-3 db to the SPL. Thus if you want maximum quiet, the 7.8” tube would be best. If you want easiest to carry, yet still be comfortable to shoot without needing hearing protection, the 6.2” tube may be for you. All .22 tubes are 1” diameter. A suppressed MkII weighs about the same as a 5.5” regular bull barrel MkII
http://www.srtarms.com