The "holes" are generally referred to as ports. They assist with recoil reduction.
Part of the sight.
Short barrels, easy to handle in close quarters. Two barrels, 2 shots fast.
Multiple barrels arranged around a common center, barrels turn around that center as they fire.
Gatling gun
From markings and proof marks on the gun barrels and the frame. In some cases you need to remove the barrels, and look on the underside from markings.
The original Gatling gun had six to ten barrels which rotated and fired independently. This was one of the earliest machine guns made.
Holes.
Interior or exterior?
Technically, there is no limit.
Dozens of countries
SteelSteel
So their souls can escape their tormented, deeply rooted prison. Or some idiot shoots holes in it with a gun.