Does it have anything to do with the internals and their resistance to force when the gun is discharged? Ie, are shotgun scopes 'beefier' in any way?
most shotgun scopes are parallax free at 50 yds and it's generally 100 yds for center fire rifle scopes.
Shotgun scopes are also good on a .22 rimfire's primarily used for hunting due to their small size and parallax setting?