I can't vouch for the 45/70 in a Marlin Micro-grove but my 30-30 Marlin Micro-grove shoots cast just as well as jacketed; maybe better. I am shooting a Lee 180 gr gas check bullet at velocities that match factory jacketed ammo (I can't recall the load data right now). Although my range is limited to 50 meters I have no trouble getting 1 to 1.5 inch 5-shot groups with my cast bullet loads using a peep sight. My .357 mag Marlin also shoots cast well. I agree with Missionary; match bullet diameter to throat size and don't worry about what type of rifling you have.
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