My experience the rifling twist that Marlin uses in the 44 mag will limit bullet weight to 250 grains or less. 300 grain bullets pattern not group.
I was told the same thing, and with a 1 in 38 twist barrel on my 1894 44 Mag I had no reason to doubt it. But my rifles favorite load is a 300 grain WFN cast bullet over W296 velocity is around 1550 fps and I couldn't be happier. I've taken a few hogs with that load, longest shot was 150 yards on a 120lbs boar.
I have noticed that with the 300 grain bullet, it only shoots well at max or very near max load. It will shoot right at 2" at a hundred yards. Its one of my favorite rifles.