AR-7.5 I intend to try, although the "lugs" will probably be pretty shallow, as the face will already be disproportionately large to house the .17 HM2 rim. It will have a faux gas key; gotta have something for the CH to grasp. A 4" long hunk of basically solid 4140 PH or AR400 (haven't fully decided yet) will have a decent mass to it. There are, of course, .22's that use much lighter slides and bolts than that of the 10/22, Marlin 60, etc. I'm sure it'll take some playing to get it right, but I'll start heavy on the springs; would rather the cartridge doesn't eject than have a case head blow out.