I use a SUREFIRE G2 NITROLON Flashlight with either P-60 or P-61 lamps.
I mount it on the P22's own Weaver underbelly base, as well as on the side of the Tac-Mount (when a Beamshot is taking the place of the underbelly Weaver base), with a Weaver Detachable Top 1-inch Ring attached to the surefire's battery-come-ON/OFF lid/switch at the back.
Fits snuggly.
Turning the flashlight body to the left (anticlockwise) turns the light ON. A flashlight helps make the P22 front-heavy (resulting in less felt recoil, which adds to accurate shooting) and is a blast to use in conjunction with a laser sight in pre-sundown light or even in pitch darkness.
Using just the flashlight beam (no laser), in total darkness without properly aiming the other night, I managed to hit a Maxwell's Coffee can @ 15m with the second shot. The first shot was used as a sighter. Having seen where the first shot landed helped adjust the second shot hitting the can dead-on, despite the fact that I was shooting from the hip.
Requires a bit of practice. It has taken me a while to get used to this technique.
Next, slap on a laser sight and repeat. It is so much easier to use the laser sight if you have mastered the flashlight-only practice of shooting like a cowboy in the dark, first.
Crete