Well with 600gr slugs use 80gr 4759 in modern light barrels,
and in barrels .90"- to .94" muzzle use 90 gr.

Here are pictures of the full bore Lee Key Slug
I had a guy cast. He took Lee Key mold and machined
it out to cast a .729" full bore slug. In picture
is a regular soft lead Foster style to compare with.
Slugs are hard alloy and can be speeded up ok.
In 2nd picture is two MRC 2 5/8" brass cases with
slugs in them. These slugs with those brass cases are
being tested first in rifled barrels in a guys 1887 WIN.
The slugs have real good shoulder to
roll crimp against when used in plastic cases.
These first ones are 500 gr. We are going to
work on another die to get the weight about 600gr,
that the brass case guys want to use.
Lee molds are not real high price so it isn't
a bad deal to get them and change them.Ed.