Hello retiredle,
Sorry, just now saw your post. What Mark Elliott said about flattening the muzzle to give a 90 degree angle is spot on. As Mark said barrels are mild steel, however if you get a good crisp edge it should last through scraping a barrel channel or two before needing touchup. I have even used a brass barrel as a scraper with good results, and both octagon and round barrels as scrapers. I have even used the barrel I was in-letting as a scraper, but this can only be done wit a straight or tapered barrel, not a swamped barrel.
In order to sharpen (or flatten to create a sharp 90 degree shoulder) I have used several methods, from a belt sander, a disk sander, oil stones, and diamond stones. I have a short cut off section of a rectangular barrel (about 2") that I mounted on a wood handle, basically just drove an appropriate sized stick into the bore. I will try to post a pic of it after Christmas.
Curtis