Good report Wachtelhund. I tried Blackhorn in my Mustang several years ago and got severe hangfires. Gave up after a few shots. My Mustang's bore is a little on the generous size also but does pretty well with that 300 grain Hornady bullet in Harvester long black sabots.
Thanks for the tip on using solder for a crush fit measuring device. That's a neat trick. Are you going to make a brass or steel shim to replace the solder?