I recently got a 20 gauge slug barrel for my rifle and wanted to take advantage of the strong action and heavy barrel by loading turned brass cases with heavy bullets. Being made for modern slugs, it has shallow rifling so paper patching is the plan: I figured why not go ahead and load it BPCR? I am somewhat familiar with it having loaded a few 45-70 cases that way. I am just wondering whether to FF or F. It is a strong action and a heavy barrel. Bullets will range from 555gr to 890.

The difference between the two powders is of course, pressure/velosity..If you are feeling you way here,I would start with 1f or canon grade. Work up a load from there.
Or, looking at shot gun loads, in drams..a black powder measurement, it would be simple to figure weight in gns to ounces, as in 1 1/4 ounce 20 guage, then apply the rule to your bullet weight of choice.