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 saw something threatening or scary.
After arriving on Earth, Starscream 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.
Really thought that one through, didn't you, Starscream?