Machine gun

Machine guns are weapons which fire small projectiles at incredibly high speeds. You would think that the Transformers would make something flashy or nuclear, but hey, go with what works.

Generation 1
Slugslinger used a machine gun, which was later modified to be controlled by Caliburst. It fired rounds of 1,200 per minute. And knowing Slugslinger, he fired it at that rate whenever he something threatening or scary.

Movie continuity
After arriving on Earth, Starscrea and Blackout downgraded their plasma cannons for machine guns (the other Decepticons presumably followed course). This loss of advanced weaponry was presumably to better disguise themselves for combat on Earth.

The Autobots, on the other hand, retained their energy weapons. Only Ratchet was shown using a machine gun.

Really thought that one through, didn't you, Starscream?