IIRC vinegar works to dissolve the crud. Failing that, remove the light's head, pull off the rubber switch boot, and loosen the 1/16" allen setscrew in the bottom of the plunger hole. That whole plastic assembly will then be free for tapping out.
What's your hourly rate for labor? If you spend an hour or two on it, you might as well just buy a 3D-cell new LED maglite (looks like the one on the right in
this picture) that will kick that incandescent one's behind in every way, better durability, better reliability, higher brightness, and longer battery life, and do it all in a smaller package than you have now.