A 12 gauge shotgun can kick like a mule, but it's generally a different type of kick, with a longer push. Yet it can definitely be sharp and hard, as I experienced when shooting Brenneke 1 3/8-ounce slugs, which produced possibly the hardest kick of any long gun I had fired at the time. And when it comes to foot-pounds of recoil, a 12 gauge can generate much more than the average deer rifle - even matching some of the large African dangerous-game cartridges.
Shotgun slugs, which are usually what will be used with scoped deer-hunting shotguns, are big and heavy. The recoil associated with them is anything but light. A 12 gauge is roughly 70 caliber - compare that with a 30 caliber rifle, and there's obviously a huge difference in the size and weight of the projectile, which of course increases recoil. (This also applies to many sabot shotgun slugs, which shoot reduced-diameter projectiles in plastic sabot sleeves of 12 gauge diameter.)
This means that a shotgun's recoil can indeed beat the heck out of a rifle scope.
The Eyes Have it
But recoil aside, the main thing to consider is that the biggest difference between shotgun scopes and rifle scopes is eye relief. A shotgun's recoil is longer than a rifle's, due to the much heftier projectile traveling at lower velocity.
A rifle scope has a shorter eye relief than a shotgun scope. This puts your eye closer to a rifle scope's eyepiece than it would be to a shotgun scope's eyepiece, in order to get the same full field of view through the scope.
With a rifle scope on a shotgun, you will be forced to have your eye too close to the scope, and when you shoot, the recoil will likely cause the scope's eyepiece to clobber your eyebrow. This is not pleasant, and often will cut the eyebrow, which tends to bleed and generally make a mess. When you spot a crescent eyebrow scar, you know you're looking at a shooter who learned about eye relief the hard way.
Your best bet is to put a shotgun scope on your shotgun, and keep your rifle scope on your rifle.
- Russ Chastain