Do you have access to a chronograph? Can you check the spread of your speeds? Is the 460 particularly sensitive to powder load variations?

How about different bullets or powders?

These suggestions would tighten up decent groups, but your photo points to something else with 4-8 inch groups like that at 70 yards.
Do you have something you can slug the barrel with to check dimensions? Any reason the bullets would lose stability at weight and velocity you are shooting?

Beyond the simple thing above, I would make sure the optics are rock solid. Sounds like you have a solid scope base and rings, how about the instillation? Is the base locked in place with something when you screw it down? (double stick tape, little dots of epoxy....? Any burs anywhere on the bottom to mess up contact? Any friction enhancer between the scope and rings? Were they lapped or was the scope bedded? How was alignment between the three rings?