I don't have a 577BP so would not hazard a guess on loadings.

BUT, I have a 577 2 3/4 750gr. that is a wonderful rifle. As Marrakai states it is the best DG caliber, ever.

It has the ME of a 500NE. Handles and recoils like a 470NE and has the .585/750 grain bullet. It will kill anything, from any angle.

Rifle: Hollis 577 2 3/4
Bullet: 750 grain Woodleigh
Charge: 125 IMR 4831
Primer: Federal 215
Brass: BELL
Velocity: 1790 fps
Group: 3.5" @ 100 yards

Rifle: Alexander Henry
Bullet: 650 Grain Woodleigh
Charge: 132 Grain IMR 4831
Primer: Federal 215
Brass: BELL
Velocity: 1940 fps
Group: 4" @ 50 yards