Im not a fan of carrying a flashlight in one hand and a gun in the other while out looking for trouble. I've done it many times and its just too cumbersome. Trying to open a door with a both hands occupied is difficult. Putting the flashlight under your arm while opening a door is a recipe for disaster. Someone on the other side of the door waiting for you can wait till you get halfway in and slam the door on you. You will instinctively reach out with your hand to keep from getting slammed and more often than not drop your flashlight.
Having done it every way you can imagine on various burglary calls, I got introduced to the weapon mounted light. The reason that many Police Depts. have gone to it is because for what they do, it is a superior method in just about every way, except one.
Most lights are small enough that they can be carried in its own holster, its no bigger than a cell phone. They are powerful enough that they are a good usefull light. They are centered well enough that if youve got someone centered in the beam, at pistol ranges you will hit them without using sights and its very quick.
You can use your other hand to defend yourself, open doors, move stuff, use your radio when the dispatchers are getting nervous about not hearing from you for a while or another unit is there communicating with you, and you arent as likely to drop your gun.
Dont take my word for it, Try it with one of your friends. Darken your house and try to find him preferably with a toy gun that wont matter if hits the floor.. If hes the least bit aggresive, he can make it very tough for you to keep hold of both things. Remember that your hands are full and you wont want to drop your only source of light or your gun.
Moving and holding a flashight in one hand and a gun in another has worked for decades, but its certainley not the best way. If thats all youve got thats what you'll have to use but I dont know of anyone that prefers that after using a weapon mounted light.
There is ONE disadvantage to not using a C or D cell maglight. Those little weapon mounted lights or small flashlights just dont hurt as much when you try to whack somone with it...
