Sorry, but I must call B/S on this statement. The only swabbing on the 16"ers was a wool fleece sleeve to the chamber only. They did have an air ejection system for the remaining propellant gasses. Source, USS New Jersey 1985-1987.
Here's a second vote for RotoMetal's Bismuth/Tin alloy. Pure Bismuth is soft but very brittle. Tap pure with a hammer and it will shatter, add tin and it smushes.
One more Q,,,is the lock square with the world, or is it slightly tipped in at the bottom. A small shim at the bottom will have a large effect at the hammer.
Short arbor syndrome. Make a shim for the arbor hole in the barrel. Try for ~.002" barrel cylinder gap with the wedge tapped into place. Also check that the cylinder face is square with the arbor hole.
Didn't know you don't have welding capability.:(
Most towns have at least one welding shop or a business that can do it. Some of the guys would do that kind of fill weld for a six pack.🍻
Weld it up, file it down, redrill from the backside. Trying to hand drill a curved surface is a pita, on the backside the bit will be less likely to wander.