A good number of 870 shotguns are set up for various scopes or electric sights.
Many of them are for use as long range slug guns.
Remington makes a special slug barrel with a cantilevered mount that's attached to the barrel and overhangs the receiver.
Brownell's sell a good number of various types of 870 mounting systems.
Generally, attaching a mount will require drilling and tapping either the receiver or the barrel.
At one time, there was a no-drill mount that was attached by using bolts that replaced the trigger plate pins, but this was not as good as an over the bore mount actually screwed to the gun.