Gene,
It shouldn’t take that much effort to open an old Boxlock.

I’d wager my left one that the firing pin on the side of the extractor that is bent in is projecting too far out of the breech face. Send the gun back to Gunter. He can likely straighten out the extractor, and set that pin to the correct projection out of the breech face. He needed to fire the gun after he worked on it, if you rushed him, shame on you, if he didn’t, shame on him. All good gunsmiths are overworked these days, I wouldn’t be too quick blame him, yet. Some ammunition has hard primers, but, never seen shotgun primers be so hard as to impede opening after firing.

Pistols, yes, that can be a thing.

He needs to see what happened.

Best,
Ted