re: .35 Whelen shooters.

When you have the gun shouldered you act as a damper and absorb the shock and vibration generated by the recoil. When the gun is rigidly mounted, two things can happen. If the gun isnt jammed into the back of the sled the rifle abruptly accelerates and then stops when it hits the buttstop and the scope guts get rattled all around. If the rifle is jammed up tight when it goes off than the recoil pad is the only dampening involved and theres nothing to protect the rigidly mounted scope from the vibration.

Good mounts and a good scope will never have a problem, but shooting out of a sled is definitely harder on the gun and scope than shoulder firing.