Pete,
I just cut my 22" 45-70 barrel down. I started at 18" and after I filed it square (as verified with a pocket framing square), sanded it smooth, and used the carriage bolt for the inner crown, it ended up at 17 3/4". I cleaned up all the scratches, etc with a bluing marker. I even epoxied the front sight back in place to make it look like a Talo Trapper barrel. Since this was my first attempt at shortening a barrel I used the 4" scrap piece of barrel to practice on.
Thanks, Dinny