The person is usually called "the starter."
A "Round" is usually how many shots are in the loading chamber. it can be one or many. A BB gun can carry 100 ROUNDS but only fires one ROUND at a time. A Machine gun fires many rounds in a "burst".
The present day Gatling gun fires a more powerful cartridge, and fires much faster. The 1800s gun was hand cranked, the present day version is powered by an electric motor. One modern Gatling gun is the GAU-8 gun on the A-10 Warthog aircraft- the gun is about as big as a Volkswagen, and fires a 30mm bullet that shoots through tanks.
A sub-machine gun fires pistol cartridges. A machine gun fires rifle rounds.
Fires more than one shot without reloading.
BB gun. Fires a round BB.
To hear the gun shot at the start of the race
The Railgun, it fires over double the speed of sound.
Only one barrel fires at a time, giving each barrel the chance to cool slightly between shots. It also fires MUCH more rapidly than a standard machine gun.
Artillery
No. They trap and slow the release of gasses after the gun fires.
The Dillon Aero M134 Gatling gun fires either 2000 or 4000 rounds per minute, it is selectable.