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.
Thundercracker (G1)'s bio stated that he uses variable-calibre machineguns. Sounds cool, aside from leaving most folks who understand how guns work wondering how you manage that.
After arriving on Earth, Starscream downgraded his plasma cannon for a machine gun, and Blackout for a slightly higher rate gatling gun (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?