The rule of thumb with weapon lights is that only SureFire is good enough. This small yet powerful SureFire M300C Mini Scout Light is designed for demanding use in tough conditions. It is also extremely easy to operate, even with gloves on.
We have come a long way from the early 2000s when the most important thing seemed to be to mount as many accessories and other crap on your service pistol as possible. Carrying unnecessary extra weight is suited for exercising but not modern military operations. Luckily we have these very minimalistic gun accessories designed with the user’s needs in mind. The weaponlight can be mounted on any firearm that has a Picatinny rail via an integral thumbscrew clamp.
Surefire M300C Mini Scout Light is very operator-friendly. It is equally easy to use with either right or left hand, with or without gloves. You get light when you press and hold the tailcap switch, and the light goes off when you release the switch. The momentary switch is an essential part of operating in darkness or low-light conditions when you don’t want to reveal your location with a constant light source. Light on, identify the target, shoot, light off, change your position!
If you need a continuous light, e.g. at a shooting range, simply press the switch as far as it goes. Press it again to turn the light off.
These models come with an M75 claw mount for Picatinny rails. Make the nut finger-tight and further tighten 1/4 turns with a rock or something. (Note for any Marines out there: it's one-quarter of a turn, not one or four turns.)
Other mounts exist from Magpul for example. The Scout light can be bolted onto pretty much anything with two UNC screws with an 8-32 thread. That's slightly bigger than the metric M4 thread. (Metric fasteners are not compatible, they are only mentioned to illustrate the size. Another note for any Marine who might be reading... On another thought, never mind.)
Alternative mounts and screws are not included.
You can find more information about the selection of weaponlights in this article.