To make the bullets more accurate by causing them to spin.
Gatling gun
Most likely a Belgian gun. Take off the barrels and look at the barrel flats (breech end on the underside of the barrels). If you see the letters 'ELG' inside an oval then it's Belgian.
The original Gatling gun had six to ten barrels which rotated and fired independently. This was one of the earliest machine guns made.
Long barreled gun, with grooves on the inside of barrel- is a rifle. Shorter version is a carbine. Handguns, such as pistols and revolvers also have grooved barrels.
Dozens of countries
Part of the sight.
Technically, there is no limit.
SteelSteel
Interior or exterior?
Any Spyder threaded barrels.
The first ones had six barrels that used .58-caliber ammunition.
OEM