Tactical lights are popular rifle and shotgun accessories for everything from airsoft to Iraq - but you have to do more than look at the photo and the price to know whether a given tactical light package is worth buying.  You must know:
      1) What will it take to mount the light on your rifle, shotgun,
             or airsoft.
      2) What comes with the light itself - and what else you'll have
             to buy to make it work the way you want.
      3) Does it run on ordinary penlight batteries - or on lithium ones?
   First, the better buys in tactical lights have integral mounts that don't require anything else to mount to any gun with a Picatinny or Weaver rail on it.  Integral mounts mean you don't have to figure out what rings or mounts to spend more money on before you can even mount the light - and hopefully batteries are included, so you won't have to spend more on them to turn it on!  Preferably, those integral mounts use thumbscrews with coin slots for very easy installation, especially if you're sending it to your kid in Iraq - rather than require any tools - and ships with batteries, like the below one sold by Internet Gun Show.
   Second, only lights which come with curly-corded remote switches to stick on your rifle's or shotgun's stock are worth buying for a rifle, shotgun, or airsoft.  (Pistols are generally poor matches for tactical lights and most lasers - if only because finding holsters for any pistol with a tactical light or most lasers is difficult.)  Look at the below one sold on Internet Gun Show for an example of the "curly cord" that runs between the actual light and the switch you mount within easy reach on your gun.  This is an expensive accessory to add later if the light package you buy didn't include it.
   Third, only lights running on lithium batteries are worth it.  Ones adapted from penlight flashlights don't put out anywhere near the light.  Ideally, the light should use two 3-volt lithium batteries; these abound for sale anywhere.
   Expect to spend $45-50 for an excellent setup for a rifle, shotgun, or airsoft.  Expect to spend much more to mount any tactical light on any handgun - as most tactical lights for handguns are only designed for one make or model, and most handguns lack the mounting rails for them.