I'll look into that matter.
tks,
jcn
?? are you using factory new shells, or reloads when this happens. I suspect either dirt and crud in the action, or perhaps a slightly deeper rim cut in the right side barrel breech than in the left one-- also, many shells vary from a few thousands in rim diameter and depth of the rim-- But as to what "Der Murph-Meister" wrote here, he is on the right path (much as I hate to give the "Devil his due")- page 40 of the 1938 WRA Salesman's catalogue- the page dealing with the Model 21--
quote: "There is no objection whatsoever to snapping a Winchester double gun and we have yet to have our first breakage of the firing pin or hammer nose from such snapping".. Let's go further- metallurgy 101 here, if you will. The firing pin is part of the hammer nose, and the entire hammer itself- it is not a free floating "striker" as the Limeys like to call the firing pin, as found mainly on sidelock shotguns-- including the American L. C. Smiths--
Winchester spec sheets for the Model 21 double would, no doubt, show that the hammers would be- as stated on page 38- quote: "Here again our chrome molybdenum alloy steel was made to play a prominent part in the design-----"!
I would test your M21 for depth of firing pin penetration by having the gun cocked, removing the forearm and barrels, and place a suitable sized block of clean soft pine against the standing breech, and then dry fire the gun, and then examine the depth of each hole made by the respective firing pin- then go from there.
If you are in TX-- Briley's or Kirk Merrington- like deciding between a BMW or a Mercedes--either choice is first class.