If your goal is to just relieve the mainsprings I use a 2" or 3" section of 1/2" solid brass or bronze rod. Slide it down the bore (perfectly harmless), point the gun upward and pull the trigger. You will hear a small bounce as the brass absorbs the impact of the firing pins. Slide the brass out. Much, much better than snap caps that lose their effectiveness in a very short time.

John McCain is my war hero.