Precision Shooter,
The capacity of the case relative to bore diameter and the sectional density of the bullet determine how quickly the volume the powder is burning in expands as the bullet moves forward. When that ratio is small or the bullet has low sectional density (light weight for caliber), expansion is quick. Quick expansion requires you make gas quickly to keep up with that volume growth in order to reach the desired peak pressure. That means either using a fast powder or using so much of a slow powder that the amount of burning surface area is so great it still makes gas fast enough to do the job, even though each individual grain is not burning at a quick rate.
In general, it is best to avoid the case packing, large quantity of slow powder solution because it can result in momentary pressures that can ring a barrel. Stick with the burn rate-appropriate powders for ease of loading and peace of mind about your gun's condition.
From shootingsoftware.com With kind permission from Jim Ristow, is an example of a commercial round loaded with a powder too slow for the bullet weight. The jump late in the pressure curve is not an actual pressure, but a wave in the steel reflecting back from a radially bulging event a ways down the barrel from the chamber. This registers on a strain gauge which measures strain in the steel, but not on a copper crusher or piezo transducer, which register only gas pressure at the chamber. It is believed to be caused by rapid expansion dropping pressure, after which the powder burning speeds up (normal for a spherical powder that has burned through it highest deterrent concentration) with the resulting gas mass and yet-unburned powder mass accelerating to catch up with the base of the bullet, rear-ending it and resulting in a momentary compression zone at the bullet base that produces the bulging and eventual ringing effect. Either going to a faster powder or to a heavier bullet mitigates the effect.
