My high-power water gun is a project that seemed simple on paper, but actually took a fair bit of doing. My school does Senior Assassin, where your goal is to hit your target with a water gun within a finite amount of time and record it as proof. So, I wanted to build a one-handed, high-range, safe water gun.
To maintain safety, I wanted the section that held the water and pressure to be replaceable or robust. I spent a lot of time dithering about pressure chambers, until I found the perfect one at Publix. A two-liter bottle is under 80 psi of constant force–and they're dirt cheap–so I can replace it after one wears out. With some custom welding and 3-D printed attachments anchored to aluminum extrusions, I had a water gun that I'm sure is going to make me Valedictorian of Elimination.