This object was made in Tinkercad. https://www.tinkercad.com/things/eC4zlB6Ko6F
Here's to everyone who didn't buy a water gun this season because they spent all of their money on a 3d printer. Instead of using a traditional hand pump method of pressurizing the water (which would be substantially less reliable when 3d printed), This version uses an aerosol can of high-pressure gas to expel the water. When the trigger is pulled, this gas will pressurize the water supply, thus forcing water out of a thin tube. This means that you won't have to pressurize the water supply manually by pumping, and also makes the gun a lot less complicated.
How to Use it
To make it printable, I split this gun into a trigger, grip, body, and intake tube. To store the water, you will need a 20-ounce soda bottle (the thread type is PCO-1881), and for the air supply, you will need a can of keyboard cleaner (connector is sized for Falcon Dust-Off cans). The intake tube fits in a similarly-sized hole inside the bottle connector, the grip fits into the octogonal hole, and the trigger fits into the small hole next to it. Make sure to nest the trigger inside the grip before attaching to the body, and use a little bit of super glue to attach each part. Pry the "cap" off the can of keyboard cleaner and, snap the it into the round connector at the end of the grip. The short plastic tube on the can should line up with the hole at the end of the trigger piece. When you pull the trigger, air should come out of the bottle connector. If so, fill up the bottle, wrap some teflon thread tape arounf the neck, screw it in, and try not to make you neighbors too angry.
How you can improve it
To the best of my knowledge, this gun is still very inefficient due to various factors, such as the size and shape of the tubes, lack of airtight seals, and the fact that the bottle connector I am using doesn't work very well. I encourage anyone who takes interest in this design to try mixing it up to try to make it better.