You used the ugly M-word -- microgroove rifling. I had a Marlin 1894 in 44 mag. Could not hit a 5-gallon bucket at 50 yards with lead bullets, but it shot 2" - 3" groups at 100 yards with jacketed bullets. Both bullets were 240 gr SWC.
If its hour-of-angle (60 times bigger than minute of angle) accuracy with lead bullets you have 2 choices: jacketed only or get a newer lead friendly rifle. My Quigley by Taylors Arms does great with AA 5744. 8# jug is almost gone
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