"Bazooka" / big rifle Needed

I haven't made a gun in a while, so I decided to fabricate the weapon of the top Mandroid (the bottom one's gun is too simple/boring in design for my tastes).



I resized your image until the Mandroid was about six inches tall, then took my measurements for the weapon directly from the sprite.



I might give it a black or dark blue paint wash, but I kind of like the bright, cartoony video game colors too.

You could modify something like a pen or marker shaft into a cannon like one of those pictured fairly easily as well. By wrapping carefully measured sections of tape, or something similar, around the main shaft, you can build up raised layers, or, alternatively, you can cut seams and details into the cylinder with a dremel or another tool to provide detail.

You could also potentially cut down a larger weapon, like the cannon on a cheap tank, to get something like that.

Check out your dollar/pound stores they often have soldier type toys with massively out of scale weapons that are usually good for 6" scale.