With less than $5, you can acquire everything you need for one gun and a whole bag full of ammo...er, mini-marshmallows.
For serious marshmallow artillery, go to the plumbing section of your local hardware store and purchase 22 inches of ½ inch PVC pipe. You’ll want this cut into one 7-inch length, three 2-inch lengths and three 3-inch lengths -- you can do this at home using a basic hacksaw, but the store may be equipped to do the cutting for you. You’ll also need two elbow connectors, two T-connectors and two end caps, all in ½ inch sizing. Spray paint and permanent markers are optional for decoration.
To use the gun, just place a mini-marshmallow in the mouthpiece and blow; the white food stuff will exit the pipe with a “flump” -- giggles may soon follow.