It sounds like you have a rough or dirty chamber... Or another problem people have is leaving excessive lube in the chamber. The lube will boil in the chamber with each shot and turn to a gummy gue first then a scorched crust making extraction near impossible. Clean and scrub the bore/chamber, lightly lube, then wipe it dry with a clean patch before shooting. Someone may have been shooting shorts or longs in the longrifle chamber so there may be a little lead ring build up. A good scrubbing will help clear this too. Start there, if the problem persists have a gunsmith inspect the chamber as it may need to have a reamer clean it up.
Edit: Oops' almost forgot in a rifle that old black powder rounds were likely fired in it. If it wasn't cleaned after use the bp may have etched a rough spot in the chamber. How does the bore look? Shiny? or Rough/cloudy? Let us know how this is turning out.
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