As country1 stated, it's not good to have no movement because when you get into the heavier recoiling calibers the recoil can damage the stock (especially a wood one), scope, rings, screws, etc. Wherever there's an action there's an equal and opposite reaction. Your shoulder moves so that reaction is dissipated, but when in a weighted down sled something has to give and eventually something is going to break when shooting a heavier caliber firearm on one.