andym79;
You say: "I am thinking of getting a 444 Marlin setup with a 1:20" barrel."
I have a 444 with a 1:18 twist on a single shot action. I call it the "44/63 Ballard" so the singleshot boys will accept my rifle more readily on their firing line.
I shoot this rifle with a 500 grain custom bullet from Paul Jones. 19 grains 4759 @ 1259 fps works for me over targets as far off as 830 yards.
Sorry, the 500 grain will not fit a lever rifle from the magazine..
FAsmus