OK, I just made one. Uses 3, AG0 batteries and a led. It is 0.27" x 0.6". The one I just made has no switch. You have to fiddle with the wire to get it to turn on. I am testing it now to see how long the batteries will last. This one is made with heat shrink tubing so the batteries are not replaceable. I could develop one that has a switch and replaceable batteries. But seeing as though it took me 15 minutes to make this one, unless I was to go into production it would be too time consuming. This one drops down a 9mm handgun. I don't have a 30 cal to test to see if it will fit into that. But it surely will fit into a .45 or .50 cal muzzle loader.