You will be looking for around 1665 fps with a 650gr projectile. I have used 95gr Varget, but prefer a slightly faster powder in my shorter barrels. Probably no help to you state-side, but 77gr AR2206 does it for me.

If it's a strong original gun (not a shotgun conversion) then 65gr of H4198 is a recommended starting load by some.

You will need varying amounts of filler or wads with all these loads.

Check this link: .577 Light Nitro