8 BORE
50 Cal.
The ball by now is so screwed up you could whistle dixie thru it.A good $2.75 screw tipped ball puller is a lot cheaper than a bench vice, barrel clamp, suitable Crescent wrench or cheater bar and strong nautical language. And a bench vice is pain to carry in your shooting pouch (I keep the puller in my patchbox and a spare in the shooting bag)
Why risk a beautiful rifle to trying to pull the breech? Chances are it will be a visible seam for ever after even if you do avoid marring the breech or barrel.
Its already got a seam, just harder to see.