65mm=2.559
2 9/16=2.5625
I think that shooting 2 9/16 inch ammo in a 65mm chamber is an acceptable option. Its pressure that you need to keep at a reasonable level and your guns should be fine. Recoil is a separate issue from pressure. Payload and velocity create recoil. Pressure is independent of those. So a fast, heavy load will generate a lot of recoil even if the pressure numbers are low.
As to anchoring power have you considered going to larger shot. Larger shot down range will have more retained energy. Anything centered will be dead. But if a fringe area hit happens a few large pellets will hit harder than the same number of smaller ones.
