Well “No effect” is not quite right. We do know that powder choice can have an affect on accuracy.
I was actually a bit surprised too, I expected that longer barrel lengths would actually favor slower burning powders. In thinking about it I have a working theory: the energy content of a slower burning powder and a faster burning powder is about the same. It is how the energy is released. It can all be released quickly or slowly, and what burn speed does is tailor the energy release to impart as much energy to the bullet as possible within a given service pressure. It just so happens most of the energy is imparted in the first few inches of travel, and adding barrel length increases efficiency but only builds a bit on what was imparted on the bullet as it immediately left the chamber. Again, that’s my working theory, I don’t have a fancy graph to support that (yet).
EDIT: As I continue to think about this, one of the measures of performance is area under the curve (Pressure over time or P/T). This is something you would see when testing using Piezoelectric inducers and test equipment. Frankly it was not something we looked at when we did load development, it is data generated but it is not particularly useful as your two primary measures of performance are pressure and velocity, you are not typically worried about the P/T curve unless you see something try odd, such as a double peak (I have seen them, and some cartridges just do it). The majority of the area is centered around the highest pressure and is best compared to as a pulse. While progressive or regressive burning powders can change the shape of this curve they are still similar. Once this pulse is over the majority of the energy has been transferred to the projectile, and the residual gas expansion and burning powder transfers a bit more so long as they are sufficient to overcome barrel friction. Again, this more or less supports my original theory that the bulk of the energy transfer happens at within the first few milliseconds and probably two inches of bullet travel when pressures are the highest. This would hold true regardless of bullet weight and barrel length.
Once we start mixing different bullet weights and barrel lengths we complicate matters a bit. Im curious on this as well, I need to see if there’s a way I can do a comparison and I’ll present the results here.
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