Hey twohats,
I've got a 14" 444 Marlin Contender barrel and I reload the 300 grain Hornady XTP with Accurate 2015 powder. This load will keep'em around 1.25" at 100 yards.
H-322 is also a good accurate powder in my barrel, if I couldn't find any 2015 I would use H-322.
The Hornady manual I have, I think it's the 5th edition, has 444 Marlin handgun data from a 15" barrel.
GOOD SHOOTING!
Slufoot