Epoxy and a steel shim as described in the FAQs, that project was started on a barrel exactly like yours, it still shoots very well at last report from Doug(canon6), I did use a very thick shim in it, .026" stainless IIRC, it originally had more epoxy in it, I removed the epoxy and used a thicker shim and just enough epoxy to hold it, regular JB Weld, not the stik. I've since switched to Superweld on the last two I've done.
Tim