A friend has a Marlin 45-70 and would like some 300 grain cast bullet loads at around 750 fps.

The lightest load listed in my Lyman cast bullet handbook is 14.5 gr Unique for 1285. I have used light loads in 45/70 using 385 gr bullets, but I don't remember any lighter than this one. If going lighter you may have to use some method of holding the powder back against the primer to get good ignition. With any such light loads with quick burning powders, you have to take extra special precautions to prevent muntiple charging.

I don't have any loads under 1200 - 1300 fps for the .45/70. Those are real easy shooting from a Marlin. I don't know anyone shooting stuff that slow from a rifle, you might have to go to one of the powder companies to find that light a load. Good Luck, Guy