Hi-Force 44 would be my first choice.

Wouldn't attempt without wire and nails. It is quick to do, so would wire the entire barrel set to be safe.

No worry about the blacking, as it was while stripping the existing that this was discovered.

Probably wouldn't even use the tire. Even if it didn't hold the breech popping open would probably be the worst that happened.

Need to remove the extension(only loose between at top between barrels) to be able to clean the joint properly. And then rinse after acid fluxing.

Mike