Before filling with amalgam, would you first perform a root canal procedure? MUST remove the rot before attempting to fill. Not certain amalgam will stick properly. If it does, well why the hell not??? Maybe the black light epoxy would be more effective? If you can’t recut the chamber to a larger cased round of same caliber as would be one of the traditional and sometimes viable fixes and the rifle is otherwise effectively useless in it’s present form, well just maybe the concept has merit.

Remember, many wonderful ideas in this world started with an idea most thought was lunacy..... Hell, many thought the moon was made of cheese before we landed people up there!!!!!!!!!