Okay. First I would like to say, yes, I have seen the plans from SA years ago where the guy built a flamethrower that could be used as a squirt gun. The only thing I didn't like about it was the need to use an air pressurizer before using it.

So I want to build one without the need for an air pressurizer. This means replicating the famous hand pump that all of us are so familiar with. I have no idea how to do this.

Any suggestions? I am still going to use PVC as its hardier and more available than most things.

tl;dr
I wants homemade squirtgun with no need for external air pressurizer.

get a scuba backplate (without bcd)

get an old water (not chemical) fire extinguisher

mount the fire extinguisher on the backplate where the scuba tank would go. this is your water source.

now get some tubing, clamps and a pipe. replace fire extinguishers tube with a longer one and put a piece of thin-ish pipe on the end preferably with a nozzle. (this is your gun barrel)

now get some bike brake parts. clamp a brake lever on the pipe. and run the the cable up to the handle thing on the fire extinguisher. rig it so when you squeeze the brake the handle thing is squeezed. this is your trigger.

get a pressure gauge, a safety blow off valve, and some fittings for hooking up to bike pump. rig them up to the fire extinguisher somehow. now get a bike pump, put a longer hose on this, and attach it to the bottom of your gun barrel. replace the T part of the handle with a dowel or something and take off the bottom. you now have pump action.

i think make mag did a similar or identical thing, find the article


tl;dr
replace the external compressor with a bike pump on the gun.

^^^ good idea. That's probably your best bet for finding one-way valves and pumps that work similarly to super-soakers.

wouldn't that take a whole mess of pumping? I guess its okay.

not unless the volume of your air tank is comparable to that of a bicycle tire.