Hey there, it wasn't a mistake — the Gatling gun wasn't technically a machine gun. It required a person to manually crank it to fire. Maxim's gun, meanwhile, was the first truly automatic weapon — it used the force of the recoil to reload the gun.

Good point about the Leo, but I didn't include it because — unlike the other builds I suggest — it doesn't have any special abilities that help to make the machine guns more effective.
At lower levels, Leo with Punishers and Avenger is still viable, but where I play, in Champions league, having an ability like stealth (ie, Inquisitor); shields (ie, Lancelot); or dashes (Strider etc) is crucial to success. If the Leo in Champions tried to just stand there firing a full clip of ballistics at the enemy, then it would be quickly and likely instantly annihilated by Spectres or Dash bots with Orks, Tarans, Sourge, or Shocktrains etc etc.