I use 21.0 grains of BlueDot in my .45-70 T/C carbine and get about 1400 fps with the 300-grain bullets. A mild load in report and recoil, it should be plenty safe in the Marlin - but work up from 19 grains just to be safe. It does not appear to be position sensitive with a regular rifle primer. It is the most accurate load in that barrel.
Expect the point of impact with any 1400 fps load to be well away from your full-power poi.
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Last edited by DoubleRadius; 12/25/08.