I managed to find some time to do some nozzle tests today. I turned a nozzle with a blank end and gave it a 15mm-3mm internal taper.
I then drilled a series of progressively larger holes and measured the range for each hole diameter. I drilled holes in the range 1mm-4.5mm in 0.5mm steps. Here is a graph of the results:
From this I can see that 4mm is the optimum hole size (as I do not have any drills that are not multiples of 0.5mm this is as near as I can get).
However, as the last nozzle diameter was 4.5mm I had to make a new nozzle to get the diameter back down to 4mm.
So here it is, the final design which gives a horizontal range of 40ft with the nozzle at a height of 3ft above the ground.
I've not had chance to test the maximum range with the nozzle angled, but 40ft ain't bad. Next I need to test the maximum range with the nozzle angled and calculate the flow rate.