Miles,
I performed a better test than the one you suggest last summer with the new 55-grain Nosler Varmageddon bullets and prairie dogs. Used both my tang-safety Ruger 77 .220 Swift with the bullets loaded up to 3800 fps (1-14 twist) and a Nosler Varmageddon AR-15 .223 with their factory ammo at 3100 fps (1-8 twist.
The difference in expansion was VERY apparent--with the advantage going to the AR-15. Ranges were from 100-300 yards, and the experiment was repeated many times throughout the day.
