Our club runs Tac Shotgun matches often a dark stage. Most of us put our regular Hand held flashlight on the mag tube with a mag tube clamp just skewing the existing clamp sideways for the match. 1 inch light like the Surefire G2 is about the same size as a mag tube. Cheap and easy solution. Very few keep a light mounted all the time. Too much stuff hanging off the gun slows you down and upsets the guns balance. Be aware that a light attached to the shotguns barrel or mag tube will change it's zero, enough to miss small close targets, re-zero after you have installed the light. This advice is for matches, only objective is to win and the targets can't shoot back.
On the beam, most indoors situations are short distance. 10 yards would be long 7 more the case at our club or in most homes. Plenty of light wide spread is what you want. No hot spots either. There is a strong tendency when shooting with a flashlight to try to center the lights hot spot on the target when what you want is the guns sights on target. Our club shoots a lot of flashlight stages indoors, beam thrown by a 6P or G2 Surefire is about right. Too much light can be a negative it can light up the gunsmoke and cause a "screen" like high beams on your car on a foggy night. Often times I will have to hold my light up at neck level and shoot Pistols one handed to avoid the smoke. Found this out the hard with with My U2 Cranked all the way up one match. Totaly obscured.
Home defense I can't recomend weapon mounted lights. To use one You have to point a loaded gun at targets that have not been identified. Could have unintended results, lots of cases were someone excited pulled a trigger by mistake. I use a G2 Surefire with the combat ring same one as in matches. Light up the possible target and if required to shoot then bring the gun to the target with the light in my hand. This is the method required by IDPA in pistol matches. Weapon mounts are not allowed. I could see a reason to use a hand held light with a weapon mounted light though. Use the hand held for searching and identification, go to the tube mount light when it's time to shoot. If so I have seen nothing better that Surefires forearm. Neat clean and mimimum upset to the guns balance.
If a person is Law Enforcement or Military this opinion does not apply. However a private citizen defending himself or home has a requirement in most if not all jurisdictions to identify a real threat before shooting. Basic safety rule is don't point a gun at anything you don't intend to destroy.
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