Removal of end cap
Most likley cause is that the batteries in the flashlight have leaked and has caused the threads on the end cap and flashlight to become fused. If at all possible, try to stand the end of the flashlight in a small plastic container with some PB Blaster, or other decent penetraiting oil, (WD-40 doesn't do very well in this situation).
After letting it soak overnight a good 24 hours, put the body of the flashlight into a vice, preferably with rubber or plastic jaws/protectors to keep from galling up the body of the light. Use a good sized pipe wrench to lossen the end cap and spray some penetrating oil onto the threads as you expose more, until loose. You will mark up the end cap, but you'll get it off. Once off, remove the batteries and use some baking soda to neutralize the battery acid. Rinse with water to clean and let dry. Remove any obstructions inside the battery tube with a wooden chopstick, or end of a wooden spoon (so as not to gouge or damage the aluminum tube) Get as much of this crude out and wipe dry.
You should be able to freely insert and remove the batteries, if not, keep cleaning the crude until the batteries go in and out easily.
Last ditch approach is to return the light to Maglight for them to repair and service. They'll return the light to you just like new, but there will be a charge for their service. Just check their web site for repair details. Read More At : Legacy X6 6V Xenon CR123A Blk...