In his book Understanding Firearm Ballistics, Robert Rinker reported that Frankford Arsenal once conducted tests of military M1 ball ammunition by tilting the cartridges upwards (settling the powder to the base) and firing them, then shaking them level (settling the powder to an even distribution) and firing them, and then tilting them forwards (settling the powder towardsthe bullet) and firing them, measuring velocity with each firing. They reported that the highest average velocity occurred with the powder positioned to the rear, the middle velocity occurred with the level powder, and the lowest velocity occurred with the most forward positioned powder. A 20 foot per second difference was recorded with each position change. (374-375)
The theory behind this difference is that air gaps between the primer and the powder can result in delayed or uneven ignition of the powder. Hence, when the majority of the powder is right up against the primer, then the majority will ignite at once, followed by a trickling down to the thinner section. On the other hand, if the primer ignition has to “jump” across a gap of air to get to the powder – as is theoretically the case when it is piled at the front of the case – then this will result in a delayed and/or uneven ignition of the powder. In a worst-case scenario, this last situation could theoretically result in a hang fire.
This difference in velocity due to powder position is why you will sometimes see precision shooters tilting their guns one way or the other before they take each shot – so that they have a consistent laying of powder in their case for every firing.
It is reasonable to assume that this issue is of greatest concern with pistol cases that have a large internal volume but relatively little powder. This particular situation with pistol ammunition is a real thing, historically, because one element of firearms technology developed while another element did not change with the times: some of these cartridges were invented in the days of black powder, a form of propellant that requires a lot of volume, requiring a large brass case. However, when the more powerful and lower-volume smokeless powder replaced black powder, the size of the brass remained the same even as shooters switched to smokeless. The .38 Special cartridge is a classic example of this instance, which is why you’ll often see load recipes with a shockingly small amount of powder going into that cavernous cartridge.
While I don’t have any scientific studies in my back pocket to prove it, many handloaders(myself included) believe that the most consistent velocity can be achieved with a powder load that fills the entire case, right up to the base of the bullet. This way, there are no air gaps in the case at all. Obviously, this amount of powder needs to be chosen by proper research and testing – we don’t just load powder up to the shoulder and hope for the best! – but if you can find a powder whose proper load volume happens to take it right to the bullet… that’s a magic spot.
