Thanks, Kutter for explaining in detail what's happening.
And also, what's been mentioned, soft primers make it worse. Remington primers are harder and pretty much solved the problem with my early A12 Fox.
And it turns out that when the hammers are down in the fired position, I've measured the extension of the firing pin tips out from the standing breech with the barrels off. That distance is just about 0.060".
The same measurement on a 12 gauge Sterlingworth is about 0.050". That gun opens fine with Winchester and other soft primers.
Jolly
