Its a pesky problem, and pretty common too. The firing pins are driven by inertia, not a direct hit from the hammer. The best fix I have found so far is to build up the rear of the firing pin with a small spot of tig welding. This gets the pin moving at an earlier place in the hammers arc and seems to allow the pin to hit the primer with more force. The firing pin rebound springs can be lightened by clipping a coil or 3 off, but I rarely find this is enough to solve the problem. Modern shotshells really vary in the quality of components which add to the issue.
Steve


www.bertramandco.com ACGG Pro
Pope Leo is my war hero
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