+1
My favorite weapon mounted light is the
Surefire Responder. This unit replaces your slide or fore-end with a compact unit sporting an integral light module. The Surefire unit is adequately protected against the heavy recoil of shotgun cartridges -- the light bezel, battery pack and light body all incorporate recoil-absorbing features to make sure that your light doesn't die. It's not just good enough to hose-clamp a Surefire 3P or 6P onto your shotgun; you have to make sure that the recoil of the gun will not damage the batteries or lamp unit.
Note that a bottom-mounted sling can interfere with the Responder unit, as the lamp assembly may get in the way of the sling. However, you might be able to drill and tap a hole in your shotgun to allow the sling to be side-mounted.
The Surefire Responder comes standard with a momentary on/off pressure switch. A nice feature about the Surefire and other weapon-mounted lights is that there are no wires hanging around your weapon to get snagged on branches, doorknobs or whatever. Everything is integral to the fore-end or slide. For an additional fee, Surefire also provdes a parallel constant on/off rocker switch, which allows you to turn the light on and leave it on without applying constant pressure to the momentary switch. Why would you want such a feature? Well, imagine that you've apprehended a goblin in your home. It's dark, so you have your weapon-mounted light trained on him. You reach for the phone to call the police. Which hand do you use? If you use your non-dominant hand, your light goes out because there is no pressure on the momentary switch. If you use your dominant hand, the goblin knows you can't fire so he rushes you. This problem is solved by the parallel constant on/off switch.
Spend the extra money, you'll be glad that you did. I use a surefire model 618fa on my 870 tactical, I bought it for $200 on ebay, money well spent. If it's not bright enough for you (which it should be in most close combat/indoor situations), you can always use a malkoff m60 drop-in which will fit this light and dramatically increase brightness.