jeweler,

If it is primer contact related, you might try a load with a Fiocchi 616 primer to confirm it. I've several SxSs' without retracting firing pins and when using some primers having harder metal in the primer cup (e.g. Win 209), I'll get enough drag to cause the difficult opening that Mike mentioned. There may be other soft metal primers like the 616, but that one has solved the problem for me in those guns where the firing pin stays in contact with the primer upon opening.

Rob H.