I have a number of Miroku guns and have found through experience that this problem is very common. You will often find that the offending (lightly struck primer)cartridge will fire in the top barrel no problem.This usually indicates a number of possible causes which may include, dirt & grease in the bottom firing pin hole, weak hammer spring, burnt and pitted firing pin nose.Also please be aware that firing pins do vary in length.
Take the stock off and clean all the components, examine for any of the above defects.Check the firing pin protrusion it needs to be 0.055" with the hammer in the fired position.Obviously check your cartridges in case they are at fault.Firing pin springs are rarely the cause of this problem, hammer springs do periodically require changing because they get tired.Parts are very inexpensive.
