To make a LPD design, you essentially build an air pressure piston water gun like Supercannon II but with a much larger air chamber. You can use bends, bushings, whatever you want, to make the air-to-water ratio higher. The idea is that the more air there is, the less effect a certain amount of expansion will have on the total pressure.
Yesterday at work I did some math figuring out how constant LPD design could be. The result surprised me. It's rather easy to get fairly constant pressure without any regulator or special parts aside from pistons.
Below is a simple chart showing how much pressure will drop for a given ratio.
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ratio %drop
0.5:1 67%
1:1 50%
1.5:1 40%
2:1 33%
2.5:1 29%
3:1 25%
3.5:1 22%
4:1 20%
9:1 10%
19:1 5%
99:1 1%
Note that you get diminishing returns fast though. I included 9:1, 19:1, and 99:1 ratios because they show how large you need to make things to get more constant pressure.
Because at higher pressures you get diminishing returns in range you can take advantage of these %drops to keep your range within a small range. For example, if a water gun with a 2:1 ratio starts at 100 PSI, your lowest pressure will be 67 PSI, which probably would only have a few feet less range.
Your water output will be affected much more than range however. But that will be minimized if you use a larger ratio like 3:1 or 4:1. 3:1 seems to be the best ratio to compromise between %drop and size.
I'm rather excited to build this type of water gun. I intend to build a test water gun of this variety for the review contest as the top prize, so if you want one for free or nearly free look into sending in some reviews.