i would imagine its 5.56, which is .223, which is still quite small for anything like a heavy machinegun which is the closest role something like this would fit. airplanes (like the AC-10 warthog for example) often have 30mm rotary cannons mounted in the fuselage, the 30mm shells fired from the full sized cannon at high rates of fire can take out tanks. in fact this drone gun looks even smaller than the M134 'minigun' you see mounted on the sides of helicopters for 'door gunners' or in the movie Predator, which uses 7.62x51mm NATO ammo, full sized bullets closer to a hunting rifle bullet than an assault rifle's intermediate cartridge. 7.62 is often used by western armies for HMGs and other larger-than-an-assault-rifle weapons like snipers, DMRs, etc. Even in the movie Predator, with a minigun modified to be carried, the actor could not carry the battery the gun requires to operate (which was wired up his pants leg) and even with an artificially reduced rate of fire (from 6,000 RPM to 1,250 RPM) the enormous backpack could only hold 4-5 seconds of ammunition supply.

maybe its something like the XM214 Microgun? which requires a team of 2 to carry and operate like a heavy machine gun (usually mounted on a tripod or vehicle)

and yea the recoil would almost certainly tip this go-kart over, especially with the gun mounted that high up. maybe if it had like ground spikes to stabilize itself it wouldn't?