Zip or pipe gun is normally a one shot usually homemade. I've seen them made out of wood, plastic and metal. For instance a small block of wood with a hole drilled in it that will hold a 22 shell, then elastics wrapped around it with a tiny nail so when you pull the elastic and nail back and let it go the nail acts as the guns hammer. Do a net search you should find plenty of them.
I am no expert but I strongly suggest if you are LE or plan to be you should be familiar with this, could save you someday.
I picked one off a kid who had made it out of bicycle tubing and a 12 guage shotgun shell. A perfect fit too. He was walking around with a ball peen hammer and I knew sure as hell he didn't have a job and in no way could be mistaken for a carpenter so I stopped him and looky looky here.
Good stop, great court time and he plead out to it along with about 4 other drug charges he picked up while on probabtion. So much for staying out of trouble while on probation. I guess his ears were full of wax when the judge released him and told him to keep clean.