[@The Survivor]

I took a look, and you seem to already have the M61 Vulcan. It's a respectable weapon, and the rounds it fires are already pretty big. [url=https://upload.wikimedia.org/wikipedia/commons/7/71/50BMG_size_comparison.JPG]The big blue round shown here is the 20x102mm round, next to some .50 BMG rounds and a stick of computer RAM for size.[/url]

But if you want to go bigger, The only thing that's really bigger than that is the [url=https://en.wikipedia.org/wiki/GAU-8_Avenger]GAU-8 Avenger.[/url] Which is mounted on aircraft like the A-10 Warthog. It uses [url=https://en.wikipedia.org/wiki/30_mm_caliber]30x173mm rounds.[/url] to some [url=https://youtu.be/33teK7L4DM4]Pretty nasty effect.[/url]