.22. Fits right on. Remove the telescoping shroud, add the back cap (the cone shaped bit) and tighten some grub screws, screw the included air stripper/bushing on the end of the barrel and then screw on the main suppressor body.
Subjectively its much quieter and a very different sound. Objectively? Not sure. My DB meter is a typical consumer one, so not especially accurate. I'll take some comparison readings and post.