I am developing a Nerf gun purely for test purposes. My main goal is to get consistent performance. I am using a few methods to do this: regulated pressure, reducing the effect of the speed of opening the trigger valve by piloting my QEV with another smaller QEV (that is in turn piloted by a smaller trigger valve), and using stiff aluminum barrel to reduce the effect of barrel vibrations.

The first major set of tests I want to do will examine fishtailing and dart stability. I want to determine when darts fishtail. I have some some simple theoretical analysis, but the results have been trivial: darts possibly are unstable when the center of gravity is behind the center of pressure. That doesn’t say that much. The center of pressure moves as a function of the dart velocity. It’s commonly reported that faster darts fishtail more easily. Is the movement of the center of pressure the main effect causing the instability here? Or does the speed cause other effects?

Towards this, I have done some dimensional analysis. Hopefully this analysis will help me decide how to conduct my experiment. I have determined that the important parameters are fineness ratio, Reynolds number, a dimensionless distance difference between the dart center of pressure and center of mass, a dimensionless mass, a dimensionless moment of inertia, and the fractional location of the dart center of gravity (or pressure)

Unfortunately, there is one effect which is hard to characterize: muzzle blast. If there’s some extra pressure when the dart leaves the barrel, then this pressure could knock the dart off balance. I can’t measure muzzle blast at the moment, but I know that muzzle blast should be approximately minimized when the optimal barrel length is used. Thus, the first part of my tests is to find the optimal barrel lengths for a variety of different pressures. I figure this is a good restriction as we’d all want to use the optimal barrel length anyway.

I intend to finish the test gun this weekend and do some preliminary tests to help figure out what gas chamber volume, barrel lengths, etc., are appropriate for future tests.

This likely will be the first test gun I build. I plan to use what I learn from this test gun to develop a second test gun. The second test gun will use higher precision regulators and pressure gauges.