I have seen an old barrel like the one shown above with staples VERY neatly fitted into it. I still can't figure out how it was done. Very machine-made in appearance. It was a 19th century barrel.
Dovetailed tenons would be used for keys as well. Lots of half stock hook breech guns of the 18th century with them.
Tenons were sometimes brass brazed on. I don't recommend it.
New England guns would position the barrel tenon between the two lugs on each rod pipe, so you have three (or four) sets of three pins. Rod pipe-barrel-rod pipe. A pretty neat arrangement, actually. I have a ca 1820 Mass. rifle like this. I can't get the rod pipes out without risking damaging the wood, so I've not taken them out, so I don't have photos to show exactly what the rod pipe tenons look like.
