Years ago I had an '86 in .40-82 with good condition rapid taper octagon barrel. Initial tests were dismal with the bullets keyholing at 50 yards. I found an article by Mike Venturino where he worked the kinks out of a friend's Marlin '95 in .40-82 and his fix was a filler. I used his suggested load of 35.0 grains of IMR3031, bulk yellow cornmeal to the base of the seated bullet, and a proper weight 260 grain bullet. My rifle went from keyholing at 50 yards to 2" - 2 1/2" groups at 100 yards. Unfortunately I was talked out of that old '86 in a weak moment, but I now have managed to replace it with a nice 1885 High Wall mfg in 1889 in .40-82 and it shoots the same load quite well. If I can find my copy of Venturino's article I'll scan it and send it to you ...
https://i.imgur.com/i7L6I7f.jpg