Thread: Nodes = waste of time.

Your rifle does not shoot every bullet into one hole. It has a "cone of fire" where bullet impacts are dispersed around the mean point of impact. It shoots groups. What causes this dispersion?


A series of factors, but the primary sources of random error at short range are the angular rate and the cross velocity. The magnitude of other factors is generally insignificant at close ranges. You can read this for more detail if you want. https://dsiac.dtic.mil/articles/rand...er-dispersion/

The angular rate is wobble around the CG of the bullet caused by things like principle axis tilt, where the bullet is slighlty asymmetrically engraved in the rifling, etc. Gas flow effects at the muzzle can also induce angular rate. The cross-velocity is a velocity vector, with a magnitude and direction transverse to the bore/line of sight - your bullet has a bit of velocity sideways/up/down when it exits the bore, due to CG offset, "barrel whip", etc. The magnitude and direction of both of these is variable shot to shot, depending on the shape of the pressure-time curve - even with extremely consistent loads there are small variations in how pressure builds over time in the bore. Because the pressure/time curve is variable, the angular rate and cross velocity are also variable (in size and direction).


You can model (you could conceivably measure) the angular rate and cross velocity, fitted to real-world data. You might find a load where, at the mean pressure/time function, the angular rate is low - a "node" for the angular rate. The cross-velocity however may still be high. They are not correlated so finding a low point for one value does not mean the other will be low.

In addition, your pressure-time function is ALSO variable. So while you may have a low angular rate at the mean pressure/time value, you will have different (higher) angular rate values for shots that have pressure/time characteristics that aren't right on the mean. Any particular load will have a range of angular rate and cross velocity values. You cannot tune it by varying powder charge to pick a low value. You have to shoot a very large sample size to even know what you have. Because different powders produce very different pressure/time curves, the same bullet at the same speed but with different powders can have quite different dispersion characteristics; 1 may be much more variable in the angular rate/cross velocity, or have much larger values.

The article I linked from Jeff Seiwert shows all of this. It is quite difficult to understand. It is most practical to not dig too deep into the theoretical stuff, and just try to measure your precision meaningfully, and determine whether it meets your requirements, and if it does not, pick a different bullet or powder. Most efforts to demonstrate anything else are just noise in poor approaches to measurement.

https://storage.googleapis.com/wzuku...nse%20Maps.pdf