Conventional high power water guns use a valve between a pressure chamber and the nozzle. Firing valves are tricky for several reasons, and it's worth thinking of ways to avoid using them. First, anything more complex than a ball valve is too hard for most to build. (Pinch valves are an exception, but they are expensive and have not yet seen widespread use, but I am certain we'll see new designs in the future that use them.) Second, the valve tends to introduce turbulence into the flow. The ideal flow path has nothing in its way, whether seals or turns. This is hard to get in practice outside of ball valves and manual pistons.
There's also the related problem of water pumps. They are hard to make well. I am not aware of any off the shelf part that can be used as a hand water pump. It is possible to make a water gun with a separate (non-pressurized) reservoir and an air pump only that will move water from the reservoir to a higher pressure chamber. So it's worth thinking about how to replace a water pump with an air pump (easily available off the shelf) and some other mechanism (possibly air) to move the water from a non-pressurized reservoir to a reservoir, or make the water flow in another way.
The simplest way to meet this goals would be to have a water gun that's like a pressurized reservoir water gun, except that there is no valve on the nozzle end, and the only valve is an air valve that pressurizes the reservoir. When the trigger is pressed, a burst of water is released, and the stream will likely continue for some time after you stop holding the trigger because the pressure in the reservoir will still be higher. The efficiency and gun response time will become worse and worse as the reservoir empties.
A relatively simple modification of this design could help. Add a small pressure chamber attached directly to the reservoir with a high flow check valve, or alternatively a valve which is either manually opened to fill the pressure chamber or automatically opened after the (air) trigger is pressed. You can allow gravity to fill the pressure chamber. Firing is accomplished via an air valve that injects air into the pressure chamber, pushing the water out.
Getting constant pressure is easy in this design; just put a regulator on your air input line. A trigger is also easy (simple air valve) as are the pumps. Overall, I can see many advantages to this unconventional design.
The disadvantages are numerous, however. First, this would seem to require air to float on top of water. This limits the angles you can fire at, introduces turbulence into the flow, and may introduce air bubbles into the flow depending on the angle. If you use a regulator to get constant pressure then you'd have to vent the excess air after each shot. That could be unacceptable inefficient, but for what it's worth, the energy efficiency of bladders is only about 65% anyway, so perhaps it could be in that ballpark. There's also the issue of the time lag between air injection and firing. Chambers which are normally pressurized would have much less start up time. It might be possible to optimize this to the point where it's not an issue.
There's another component missing from these designs: preventing water from leaving through the nozzle. You need some sort of stopper or valve to do this. A firing valve would reintroduce all of the problems with valves that I'm trying to avoid, which is no good. The Buzz Bee Drench Force
uses a stopper that moves out of the way when you fire, which seems like the simplest approach. The good news is that because the pressure is low, you don't need a particularly good seal. I would personally use something sliding rather than rotating, but perhaps Buzz Bee considered that and it did not work. One disadvantage of this component is that it limits you to just one nozzle size.
I could think of other ways to accomplish this goal as well. In particular, I spent a lot of time years ago designing a water gun that runs entirely on compressed air that used a complicated and expensive set of special air valves in addition to an air cylinder [I edited this sentence a bit to clarify what I meant better.]. It was rather fancy and designed to recycle some air to improve efficiency. Ultimately I decided that is too complex and probably wouldn't work that well anyway. I've also been toying with applying some valvelesss pulsejet
concepts, but I don't know a lot about pulse jets at the moment and am not certain this would work.
Let's use this thread to discuss any unconventional or speculative water gun designs that may be worth building.
Edit: For the lack of a better name, let's call the design I mentioned AIGF (air injected, gravity filled) until someone can think of a better name. I also just had the realization someone might be able to make the Drench Force style valve work at higher pressures. If that's possible then that would be a good valve to use for water guns. No turbulence introduced in the valve. The manufacture might not be so hard either. You just need to seal a flat surface under pressure. I can think of one fast opening mechanism, too, though closing fast might be an issue.