The lights often equipped on those rails are usually fairly dim. They're usually modified small Mag-Lites. Powerful for their size, but still not the light that a big police flashlight/club can give . . . although what comes after the slash is really the main reason those flashlights are so huge. The main reason for them being so dim is battery size, such a small battery can put out bright light, for maybe an hour. Or, a lower lumen level for several hours. The reason they've gotten as bright as they have in the last 30 years is vast improvements in LED technology to create lights with much less heat wastage, and so can actually provide a higher lumen count at lower amperage because in LEDs light is not a byproduct of heating a filament.
Anyway, the light from a handgun mounted Tac Light is about the brightness of the personal lights already used in the game. Although, the Tac Lights you can mount to a decent rifle or shotgun can be a good bit brighter.