I don't know a lot about velocities, energies etc but for me I wouldn't be too comfortable at 100 yards with an open double. The one I just shot was with mt PHs 500 double at about 25 yards. The accuracy you are getting sounds bad for any range. If it were me shooting a 500 I would want full power and penetration from it. My hunt certainly gave me a new appreciation for a powerful double. The cover and style of our hunt meant a shot of even 50 yards would have been unlikely. Hopefully someone can chime in regarding penetration and velocities. Is reregulting with full power loads an option?