This will obviously show what is written above, but whe you're done you'll have a blast! :)
FOLLOW THE INSTRUCTIONS beforeyou start to build. I take no responsibility if you somehow hurt anything with this. Thanks, and have fun.

Step 1: Materials and Things You'll Need

You will need:
cardboard,
an adhesive, I used tape.
4 (or more) paper towel rolls.
one of those smaller, but thicker, paper towel roll look-a-likes (for the back handle)
an empty 2-liter
an empty water bottle,
an empty crescent roll container (or something to use as a handle)
a wooden stick (optional if you want a little more detail)

and of course, some spare time. :)

Step 2: Reference Pictures.

You'll see pictures below, these are what I used for reference for the mini gun.

Step 3: Onto the Building!

Don't look up here. :p
Just look at the pictures below for info and guidance.
This step is all about the barrel.

Step 4: The Body and Ammo Container.

Again, looking below helps. Take what you have so far and add a handle to it. Do this by taking a strip of cardboard 4 layers thick, cutting it half, and adding it to your empty crescent roll tube. Then add to your mini gun.