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Read as many of the reloading articles as you can.
Much wisdom there and MANY real world scientific (and some not so) tests.
As for the Hornady 265 FP a standard load that seems to work well for most is
46 grains H-4198, WLR primer, Remington brass, crimp in the cannelure.
This gives in the mid 2200's fps and good accuracy.
With the 240 Sierra, try 45 grains 4198 (either variety).
Nice easy shooting load but don't expect that bullet to hold up after hitting anything going over Mach 2.
Most of us eventually graduate to the big WFNGC and LFNGC bullets that allow the 444 to release it's full energy potential.
If you show up over there and show some interest, I will send you some 310 grain Leadheads WFNGC to play with.
If you have to have just one powder for the 444 it would be H-4198.
The IMR version is a little faster and does not have the temperature compensation that the ADI/Hodgdon powders do.
The Hodgdon version also meters much better. Recommended.
Other good powders are Reloader-7 (a little slower), H-322 (slower yet), and H-355 (slowest of all).
The latter 2 are usually used with bullets over 300 grains.
Don't believe what you may read about those big bullets not stabilizing in 1/38" twist.
Driven as fast as possible (those lead bullets have much lower friction) they stabilize just fine up to 355 grains.
They also want to be sized .432" for use in the microgroove barrels.
More good reading from one of our premier WFN bullet makers and a 444 nut:
Beartooth Bullets > Tech Notes > .444 Marlin- America's Most Versatile Big-Bore Part I
Beartooth Bullets > Tech Notes > .444 Marlin- America's Most Versatile Big-Bore Part II
Beartooth Bullets > Tech Notes > .444 Marlin- America's Most Versatile Big-Bore Part III
To tide you over here is our Master Reloading Table which is curated by yours truly.
The PDF converter in OpenOffice doesn't work very well so there are some blank pages.
The original document is at:
Dropbox - 444 Master Reload Table revised Nemo-7h.xls