Mark Rober converted a leaf blower with some PVC plumbing parts and made a 13 rd snowball machine gun. The set up is rather simple, he uses a plumbing Wye between the barrel and leaf blower hose. At the wye junction he adds a speed loader magazine. It is somewhat similar to a shotgun speed loader. It is a tube with ammo and a push rod to feed the ammo into the chamber. When he runs out, he just grabs another magazine and plugs it into the wye and continues to unleash frozen mayhem.
Here is a video by Rober detailing how he made the snowball machine gun.