Print a plastic water pistol that works...This will impress me and it is doubtful it can be done at this point. Plus how strong and durable are the plastics that are printed compared to the usual injection molding
This is not true. The additive manufacturing world is 20 years old, getting very large, and includes many different technologies.
The very cheap machines that are coming out for home use print plastic, yes. But step up a grade in quality and price, and you can buy machines that can print in metal, ceramics, sand, glass, and even wood. I don't see anything in a gun that someone could not make on a current metal printer, except maybe the spring.