The CX will be very effective up to the distance, point the velocity drops below a point that allows adequate expansion. You can do your own test I suppose. Tough I would research velocity loss at distance. Example, if you have 3000 fps muzzle velocity with a 90 grain CX. Near sea level this 3000 FPS muzzle velocity erodes to a little over 2300 FPS at 300 yards. What I would call attainable velocity in a 22" bolt action rifle. Attainable muzzle velocity from a 18" gas gun load would be lower than listed above, we may expect.
Randy suggest a minimum Impact velocity of 2300 FPS with the Copper bullets as a rule of thumb. I'll agree with this lower threshold. With the example above I would expect that 90 gr CX to offer good terminal performance out to 300 yards. This is providing you have the capability to deliver the bullet in the vital zone in your field conditions.
As your muzzle velocity varies or your altitude varies adjusting your distance threshold can be adjusted accordingly.
Randy suggest a minimum Impact velocity of 2300 FPS with the Copper bullets as a rule of thumb. I'll agree with this lower threshold. With the example above I would expect that 90 gr CX to offer good terminal performance out to 300 yards. This is providing you have the capability to deliver the bullet in the vital zone in your field conditions.
As your muzzle velocity varies or your altitude varies adjusting your distance threshold can be adjusted accordingly.