There is, of course, a big difference between touch holes as would have been found, or rebushed, on American longrifles and those gold or platinum lined ones on English guns. The English ones aren't screwed in, they are swagged in place, frequently through a hole in the far side of the barrel that was subsequently plugged. The inside of the swagged bushing was cone shaped like the inside of a White Lightnin' liner. Some were swagged in with a tool that could enter the barrel from the breech but most that I've seen were done from the far side of the barrel. Doing this requires very malleable metal essentially 24K gold or the platinum. Alloyed gold, like 14 K, will work harden and crack when swagged out. When you consider the nice flange on the bushing to protect the side of the barrel one needs to start with a substantial sized gold or platinum rod to make the bushings. In Neal and Back's Manton book, theyu report a record of Joe Manton selling back to his platinum supplier scrap platinum left over from making bushings. I forget the amount but remember that it was very large.
Tom