It would seem that you have eliminated firing pin drag as the cause of difficulty in opening your gun. None of us can really know just how difficult it is to open, or if it is simply normal cocking effort for this particular shotgun. I think I would next try dry firing the gun with no snap caps or fired shells in the chambers. If the effort required to open it is still the same as it is with fired shells in the chambers, that would completely eliminate the brand of ammo as the cause.
So then, the difficult opening might be the perfectly normal cocking effort of compressing mainsprings, etc., or it could require more force than normal due to other factors such as being gunked-up with old congealed lube, or a lack of proper lubrication, or rough rusty surfaces on the action knuckle, hook and hinge pin, hammers, etc. Nothing wrong with trying a different brand of ammo if you really need it and it solves the problem. But I'd rather do a bit of additional inspection and trouble-shooting first to confirm or eliminate other possible causes.
