You can black the edges of your scraper with a marker and push it into the barrel, twist the scraper in the chamber, remove the scraper and see where it is rubbing against the I.D. of the chamber. File the areas that were touching, reblack, and do the process again. It is slow, but you should be able to profile the scraper much like the process you inlet a part into a stock, but reversed. When you have the metal rubbing all the way around the scraper, you have the profile.
Roger