For a weapon light, I would recommend something that only has one single mode - high. Along with that, it should have a momentary on/off switch, as you do not want to have the light on except for the moment you are about to confront the enemy with it.
Also, if this light will be used indoors, be careful of the light being too bright and having the light bouncing off the white walls and coming back at you, hurting your own night vision or even temporarily blinding you as well as your enemy.