Hi Heath,
Have one of those .444 Marlins & wouldn't swap it for anything. Not joking, it shoots anything from Lee 200 gr. Flatnose Plainbase up to those big 300 gr. Lee bullets. It has the slow 1-38" rifling but still shoots the 300 grainers. Just been using water-hardened wheelweights dropped from the mold, Lee Liquid Alox, sized .430". Used some .429 bullets that I bought from a company in Oregon and they leaded like crazy so don't go smaller than .430" I'd say, maybe .431 would be better? My favorite load is 20.5 gr. BlueDot and the Lyman 429244. My son loves those 200 gr. Lee's with 14 gr. BlueDot. Seated the little 200 grainer to 2.430", the Lyman to 2.545", and the big 300 gr. Lee to 2.500" and have no feeding problems in my Marlin. Use the Lee factory crimp die and all is good. No filler of any kind and magnum primers.
Hope this helps.
Newtire