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I couldn't tell you that. You have to calculate what size the optimal nozzle would be using data from your other nozzles. It is different for each gun, especially modded ones.

Yes, measurement is necessary (unless you're very comfortable with CFD). You can then plot the data points for range and nozzle diameter and see a trend. Note that pressure must be constant for all tests or else this analysis is meaningless (most people don't have to worry about this but if you do...).

If you're more mathematically inclined the trend should fit an equation like range = A * e ^ (- B * (d - C) ^ 2) where d is the nozzle diameter and A, B, and C are constants determined from the data. C would be the optimal nozzle diameter. This part isn't necessary at all if you can just plot the numbers on a graph but some might find it interesting.