BP is the way to go.  The tapered or funnel-shaped barrel is a challenge.  I recommend using only nearly pure lead bullets for experimenting.  Find some 45-70 bullets & try them.  I would convert some hollow-base form with a drill press.  Drill some of them very deeply to let them collapse inward instead of transferring the swaging force to the old barrel.  I have shot a lot of bullets into a box of dry sand when I wanted to inspect the bullets after firing.  About 3 feet of sand will stop most bullets.  The sand will scratch the bullets, but I would only be looking for gross diferences; end-on hits vs. keyholes and what the bullet bases look like after firing.  Someone else will say sawdust or something else.  I have only used sand.