Big Gunz are Ork field artillery units, commonly crewed by Gretchin.
They are usually large guns mounted in a cannon-style, wheeled for transport and low to the ground so their Gretchin crews can access them. This concept was no doubt copied from encounters with Imperial artillery units, but Big Guns teams are far less disciplined then their Imperial counterparts. Big Gunz are used to pound enemy lines into submission and destroy enemy vehicles from a distance as Boyz close in.
They are often used in groups and have a single team of gretchin crewing up to three of the guns at once under the supervision of a Runtherd. Grots commonly fear the prospect of being included into Big Gun crews and some Runtherds often use the threat of serving in a Big Gunz battery to to keep the rebellious Grots in line.
You can also find out how I painted the Krew.
As for the gun that will be undercoated black. I do like the Big Gunz models that have been produced for the Orks, I don’t like the lists that all the Big Gunz have to be the same (though tactically I know that is better). I already had a Kannon so was pleasantly surprised when my mother in law for my birthday gave me a Zapp Gun. It was easy to put together, though no instructions where to put the extra bitz, so they could be in the right place, more likely they are in the wrong place…
It comes with two krew, but in reality you need more than two as they die very easily at which point the gun becomes useless.
I then purchased two more Kannons. These were painted black and then drybrushed with Tin Bitz and Boltgun Metal before some of the details were added using red, yellow and green.
This is a picture of them in action.