Part of the sight.
Multiple barrels arranged around a common center, barrels turn around that center as they fire.
Short barrels, easy to handle in close quarters. Two barrels, 2 shots fast.
Gatling gun
Ribbing is a knit fabric that stretches a lot and is usually used at the neck opening of T-shirts. Ribbing is also used at the wrists and waists of sweatshirts as well as at the ankle openings of some sweatpants. Elastic can be used instead of ribbing in some instances.
The "holes" are generally referred to as ports. They assist with recoil reduction.
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.
Dozens of countries
Technically, there is no limit.
SteelSteel
Interior or exterior?
Any Spyder threaded barrels.