I've fire-lapped several guns, both black powder and cartridge,with fire lapping bullets from Beartooth Bullets.

The worst results were "pretty good", the best were"outstanding".

Had a .357 Mag Ruger stainless Blackhawkrevolver go from 6 or 8 inch 25-yard groups to 11/2 inch25-yard groups. The gun had a bad constriction where the barrel screwed into the frame and the bullets were being swaged down by it. Fire lapping eliminated the constriction but it took about two dozen lapping bullets with the stainless barrel. Hada blue Ruger Blackhawk .41 Mag go from 12" to 6" 100 yards groups with a scope from a bench rest. It took only a dozen fire lapping bullets.

Fire lapped my Lyman Mustang and shaved just an inch off 100 yard groups with the same bullets and loads, BUT gained a whole lot in ease of loading and cleaning. It really reduced fouling levels.

If you haven't already seen Beartooth's technical notes on fire lapping muzzleloaders, you need to give it a read. Link is below.

http://www.beartoothbullets.com/tech_notes/archive_tech_notes.htm/48