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.
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.
The original Gatling gun had six to ten barrels which rotated and fired independently. This was one of the earliest machine guns made.
Interior or exterior?
Part of the sight.
SteelSteel
Technically, there is no limit.
Dozens of countries
Any Spyder threaded barrels.
The first ones had six barrels that used .58-caliber ammunition.
OEM