22 Mag in 22LR
Will has it right.
Ruger (and maybe others) manufacture a single action revolver that can be purchased with an extra cylinder for the 22 Mags. The barrel has to be sized for the 22 Mag projecticle, which had a diameter of .224". The 22 LR ammo has a diameter of .222", but still shoot reasonably well in the .224 barrel.
With the 22 mag ammo and cylinder in place, my 6" Super Single Six is a tack driver!