I filled a standard nozzle with epoxy resin and drilled it out to have a 1.5mm diameter hole. I was hoping that might increase the range but the more I read about hydrodynamics the more I believe it will actually reduce it.
This is because the narrower diameter will resist the flow of water through the nozzle. I'm going to try it as soon as possible.
Super Soaker Central recommend a tapered design for maximum effect but these are not readily available so I've decided to make one.
I've order some 22mm diameter Delrin bar which is easy to machine and I intend to use a tapered reamer to obtain a conical internal diameter.