That Machine Gun looks great! You might try YouTube and look for some of the steampunk people who create props (itsatrapUK, is one I know of). They do a lot with PVC and I am thinking PVC tubes around a core, if painted well looks like a gatling gun.
My dad ran a division of a paper company, they made tubes and various shapes for paper furniture (he had a patent on paper furniture, if you sat on a bar stool or office furniture in the late 60s, 70s and early 80s, it was dad's paper). Anyway, he was very creative with paper. I had loads of toys made from paper and embelished with paper mache. I had a mortar that looked like the green plastic army men, a bazooka, german potato smashers, greek/roman armor, shields, swords, spears, etc. I celebrate Talk Like a Pirate Day (September 19, if you are wondering) I make various things (usually leather) to raffle off and money goes to a no kill shelter here in KC (although one year, we found a parrot sanctuary in need of donations). Anyway, I got a paper tube and other bits of cardboard and put a ships cannon together. On an early morning commute, i did not get tailgated with this in the back of my truck. Parents and grandparents really bought tickets for their respective little pirates. The Grandmother that won it told me just last year the problem was her three grandkids all thought it was their personal one and did not want newer grandkids playing with it. Ha. You can do a lot with paper mache and cardboard. You don't want it out in the rain though.