Something to bear in mind when hand loading is the amount of power you’re going to get out of an amount of powder will be dependent, in part, on the caliber of the barrel, thanks to a concept that has several names: “volume ratio” and “expansion ratio” are the two most common terms applied to it.
Bullets are propelled by cartridge pressure, which is calculated by the following formula: Pressure = Energy/Volume.
Volume is described as the amount of space in the bore from the cartridge base to the muzzle. The problem is that the amount of volume changes as the bullet travels the length of the barrel. And, of course, the length of the barrel will vary from gun to gun… bringing us to the concept of this article.
Let’s use the popular Federal 230gr JHP Hydra-Shok .45acp as a sample round. As tested by the website Ballistics by the Inch (http://www.ballisticsbytheinch.com/45auto.html), this ammunition will accelerate from 0 to 754fps within the first two inches of barrel and more-or-less stabilize at 895fps at the 5” inch mark. This makes sense because Federal most likely built the round with the intention that it would be shot out of 1911 Government models, so they did their homework and gave it a fast-burning powder that paired well with this bore and five inches of barrel.
But we’re not stopping at 5 inches of barrel….
Over the next 11 inches, our Hydra-Shok gains only about 100fps for a whopping 9fps gain per inch of barrel. At the 17” mark, the Hydra-Shok’s velocity drops to 955fps. At 18” of barrel, it drops even further to 937fps.
This velocity loss is occurring for several reasons. First, the .45 ACP is a low powder volume cartridge, so there isn’t a ton of energy to begin with, at least relative to what would normally be fired out of a rifle-length barrel. Second, the wide bore of the .45, combined with the increasing length of the barrel, has created a situation where there is so much bore volume in place, the amount of pressure being exerted by the burning (and dwindling) gunpowder can no longer overcome the friction of the bullet against the barrel.
This phenomenon is most apparent in larger bore rifles – this is why you don’t often see .44- and .45-caliber rifles barreled longer than 16- or 18-inches – but it applies to any bore. Remember, pressure is based on energy to volume, so if the energy (aka, amount of powder) is not appropriate to the bore, then the example of our .45 Hydra-Shok can be repeated with a smaller caliber, as well. For example, look at the .25 ACP data at Ballistics by the Inch (http://www.ballisticsbytheinch.com/25auto.html) to see how that round caps out at 14-inches ofbarrel and consistently loses velocity from there on out.
Now, this isn’t to say that you can’t design an efficient pistol round for a long barrel. What it does mean is that you need to be aware of this concept when hand loading for this type of situation. Namely, choose your powder accordingly: a slow-burning pistol powder may well turn a relatively low-volume cartridge into an effective carbine round. You just need to plan and execute accordingly.
