I cast the RCBS 405 gr bullet (gas checked) and load with WLR primers and Reloader #7. You can easily get velocity up to 2,000 fps out of an 18" barrel with this powder/bullet combo, but my go-to load is right around 1,600 fps. Anything more than that, and it gets really unpleasant for me to shoot, and I seriously doubt if any bear or moose would know the difference between 1,600 and 2,000 fps at 50 yards or less. I previously tried most of the powder listed in this thread, but RL #7 seemed to perform best in my Marlin. Might be our consistently cooler temperatures or maybe even the rifle itself. Who knows, but it likes RL #7.

My 45/70 is not microgroove, however I do have other Marlins that are. As previously posted, properly sized bullets are key to accuracy with these barrels. I would not hesitate to buy a MG Marlin for cast bullet shooting.

I will mention one more slightly off topic thing about Marlins. If despite your best efforts, you just can't get it to group cast bullets worth a darn but it seems to shoot jacketed bullets just fine, then you need to check the barrel for a slight choke or restriction wherever there is a dovetail cut into the barrel. If present, it will NOT be visible to the naked eye, however you can find it by running a tight patch down the previously cleaned barrel feeling for an increase in effort, or some resistance to the patch and marking your cleaning rod with a sharpie when you feel that constriction. It's usually the worst at the Magazine hanger dovetail, but also noticeable at the rear sight dovetail. I have noted this defect in a number of Marlins over the years, both with Microgroove barrels and modern ballard rifled ones. This constriction does not seem to have any detrimental effect on jacketed bullet accuracy, but if present, really opens cast bullet groups up big time. The good news is, that if you do have a constriction it can be cured with less than 50 rounds of fire lapping using reduced loads and cast bullets.