Thanks all for your replies.
It's an old barrel I had given to me, not fit to be used again. I stripped all the usable stuff from it, the pipes, sights, etc.
Think I chuck it up in my lathe and practice cutting internal threads and fit the breech plug again.
Saw on Amazon I can get a 11/16x20 tap for cheap and run it down on the grinder to make a bottom tap. Be good practice for me.