Ghost Ring Sight

[MMH]

What is it? Just a peep sight for a shotgun?

[Hawk]

What is it? Just a peep sight for a shotgun?

Short answer - yes.

From Wikipedia:
Ghost rings
For many shooters, the ghost ring sight is the fastest type of aperture sight. It is fairly accurate, easy to use, and obscures the target less than nearly all other non-optical sights. Because of this, ghost ring sights are commonly installed on combat shotguns and sub-machine guns (and to a much lesser extent, rifles). The ghost ring is a fairly recent innovation, and differs from traditional aperture sights in the extreme thinness of the rear ring, and the slightly thicker front sight. The thin ring minimizes the occlusion of the target, while the thicker front post makes it easy to find quickly.
A ghost ring can also take form of a tube ranging from 3-6 cm long, with the post mounted at the inside end of the tube. This type of ghost ring is used when there can be no front sight at the front end of the barrel. It is slightly slower to use, because the shooter's eye has to focus close on the post, then the target. However, in situations when extremely fast sighting is required, the shooter can simply look through the tube, and it is simple to see if the view is straight down the tube.