Shorter bbls in 308 or whatever... Will need to pay attention to down range velocity and the bullet construction. If you get down to 16 or 18" bbl, MV is going to suffer a decent amount. Reduced MV means reduced velocity down range. A stout bullet like just about anything bonded, or even a 165 gameking (in 308) may not have the velocity and energy to expand well at the longer distances you mentioned. Just be mindful of that. You may want to play with the expected velocity numbers and choice bullet parameters in a ballistics program to get 300+ yard velocity estimates and cross reference them to minimum velocities needed for adequate expansion
