I purchased this for my 9mm carbine, and it works perfectly. Once inserted into the bore, the laser shines from the end of the muzzle, allowing you to line up your sights to the red dot. It is very bright. The unit takes two small watch-batteries, which are inserted from the back, using a screw in cap. This also came with two spare batteries. Screwing the back on turns the unit on, once contact is made with the batteries, which works fine - once finished, just unscrew the back to turn it off. I prefer to take the batteries out completely when not in use, because it is a bit sensitive and the laser can come on even when the back is loosely connected. Great device, extremely useful!