Instructions:
http://churchfun.com/2008/05/26/how-to-make-a-marshmallow-gun/
or http://www.ehow.com/way_5923191_pvc-pipe-marshmallow-shooter-instructions.html

(Use Schedule 40 PVC pipe for the barrel and thinner wall stuff for the rest. The thinner wall is a little bit cheaper but most importantly it is easier to cut.)

This is a fun toy that students make. Have a contest to see who can shoot a small marshmallow the fastest without cheating. Use a photogate or a "Chrony" (from E. Author Borwn Company) to measure the projectile's speed. Then have an unlimited contest where students can do anything other than use compressed air to shoot it anything soft out of the barrel.

This is a nice, simple, high interest, experimental method activity. Pressure fit all the pieces. Do not use the PVC or CPVC cement. This is very harmful in spaces that are not well ventilated -such as classrooms. I only use this stuff outdoors and only when I need to. In hailing the fumes can stop your heart. It does not clean well. Wear gloves and old clothes if you are going to use it.

Guns are not allowed in school, so they must be called toys. Get the principal’s permission before pursuing this activity. I limit construction to my classroom.

Have let students keep the muzzles and make any modifications to them outside of class. You hold onto the body for safety reasons. Change up contest and focus on the projectile. Let students change the projectile to maximize the muzzle velocity. Then regulate the pressure of the launch system. Instead of using a person's breath, have the students make the pressurizer and redo the competitions.