Ever see any one get a black eye from not properly holding their scope equipped rifle? The recoil leaves a nasty shiner. Our squadron safety officer sported one for about six weeks after open day of hunting season, when asked...Don't Go There! LOL
Though the bullet may not penetrate the multiple lenses inside the scope tube, the kinetic energy of the bullet, especially a 170 grain or better 30 cal would cause the scope to fragment and even drive the scope backwards into the skull of the shooter enough to cause significant blunt force trauma to kill. The brain is a well insulated and resilient organ, but a well placed sharp blow can have fatal consequences.
**although the bullet is 30 caliber you have to remember it's a full metal jacketed bullet. They don't usually blow up at the velocities shot out of a 30-06 or a .308. Tobe exact they don't deform much either.
Lets not also rule out the bullet itself. It could fragment upon impact with the front lens and spray bits and pieces into the face and head of the opposing shooter too. Render him blind, you remove the threat. There are some vital places on the skull and neck that if punctured by a bullet's jacket could cause enough blood loss to kill quickly.
**rifle bullets used by the military snipers are fairly heavy jacketed, and have a C/G error in the .00025" range. As long as the bullet is stablized in flight; they know just about exactly where it's going as they squeeze the trigger.
Remember in the Civil War, snipers were accurate, but the weapons they used to kill were less effect than those of today. Most times the sniper just seriously injured the person he was shooting at. Blood loss and infection usually are what did the actual killing.
**CSA used a lot of Whitworth target rifles for sniper's weapons. They were good for at least 700 yards, and many one shot kills were recorded at 850+ yards. (Gen. Sedgwick took a head shot at 850 yard during the Battle of The Wilderness)
Just because it can't be replicated doesn't mean it didn't happen. Many things in our lives happen but can't be repeated. I once saw the after effects of a tornado that hit a drive in restaurant. The plastic drinking straws were driven into a telephone pole!!! They looked like porcupine quills sticking out of it and when you tugged on them, you couldn't easily pull them out. Try replicating that!