I have the same problem with my reloads using Winchester primers with my early A grade 12 gauge Fox. It too is hard to open after firing these shells.
Shooting reloads with Remington, Fiocchi and Cheddite primers, the gun opens fine.
I measured the firing pin protrusion through the standing breech with the barrels off and both hammers down in the fired position. The firing pins are at the tips of the hammers.
Both firing pins measure about 0.060" extending past the standing breech. Other Fox's that I shoot, they extend about 0.050" and they will shoot those Winchester primed shells just fine.
With Fox doubles with the barrels off, it's easy to dry fire it and make that measurement. Put the barrels and fore end back on and recock the hammers. Can't do that with some guns, ala L. C. Smith.
If you can measure how far the firing pins extend past the standing breech on the problem gun, that may tell you something.
Annoying to say the least.
