An autocannon is a fully automatic projectile weapon which fires shells instead of bullets.

20x110mm autocannon round.

I don't think it comes with the model, in the game I almost positive you have to pay the points for it.

Personally, I don't like the Autocannon, it's only AP4, It can't even take a space marines armor out and it's only 2 shots, I would much rather have a twin-linked heavy bolter or something more purpose built.

You are probably thinking of an externally powered machine gun, such as the 25mm autocannon on the M3 Bradley. However, manually powered chain guns, such as the Josselyn Revolver were patented in 1866.

The Denel NTW-20 uses a 20x89mm cartridges, while the Denel NTW-20x110 and Croatian RT-20 both use a 20x110mm autocannon cartridge. A prototype 25mm sniping weapon has been developed in the US, but is not currently in service.