My rifle looks to be microgrooved. I have read that some 44 ammo can tumble out of the barrel.
I had a poor experience with lead and microgroove barrels. Lead bullets did strip if I pushed them too fast. I tried some 290 lead and that was a scattergun experience.
I had the barrel changed to a Ballard barrel, it handles lead better.
I am of the opinion that Marlin needs thicker lands and deeper grooves and a faster twist in their 44 Mag barrels. But when you talk to them they say they are following SAAMI specs.
Stupid is as stupid does.
I also got blown groups in my microgroove Marlin 336 when I pushed lead bullets above 1600 fps.