An older gun smith in this town, now passed, sectioned a TC breech through the thin web that fit over the lock plate. In that web, he just happened to cut through a void, (casting flaw pocket) in the breech. That was about mid 1980's. I saw the breech, (YIKES) and wondered at my own shooting of that rifle, back in the 70's.
Had that rifle been fired, it most likely would have blown the lock out of the gun.
Edited: IIRC the WEB was almost but not quite .060" and the skin of the fault above and below the fault was 2 thou. (.002")