Reloading: Pressure Curves and Powder Charging
Tom McHale 04.01.14
Last time we dove into reloading, we covered the re-priming step of ammunition reloading. Now it’s time to get to the pyrotechnic part: charging the cartridge case with a new load of powder, or technically speaking, smokeless propellant.
“Charging” is such an aggressive word, isn’t it? All it really means is “filling.” It’s not as intimidating as it sounds. For purposes of reloading, charging simply refers to putting new powder back in your empty cartridge case.
It’s critical to use the right powder, and exactly the right amount, as they are all designed with different burning rates dependent on the cartridge and projectile type. Burning rate is exactly what it says—an indication of how fast the powder itself is consumed in the burning process.
Sometimes beginners equate burn rate of a powder and expected velocity of the bullet when fired, and that’s not exactly how it works. To understand a little more about powders, you have to understand the concept of a pressure curve. Let’s look at a simplified description.
When powder ignites in an enclosed space like a cartridge, the chemical reaction creates a volume of gas. Pressure comes into play as the expanding volume of gas has nowhere to go, so pressure increases. At some point, the pressure of the gas exceeds the force of friction holding the bullet in place and the bullet is driven out of the barrel.
However, there can be too much of a good thing. While increased pressure can create increased bullet velocity, there are a million “it depends” exceptions—and this is exactly where the concept of pressure curve comes into play.
When the firing pin smashes and ignites the primer, it lights the powder in the cartridge, and a very rapid burn and gas expansion occurs. Pressure builds very, very quickly. As the bullet starts to move, pressure falls. This is because the space available to the expanding gas becomes larger as the bullet travels down the barrel. After the bullet leaves the barrel, pressure falls to zero, as nothing is left to contain the gas.
Powder burning rate is somewhat counterintuitive. You’ll find that small pistol cartridges, like .32 ACP, use faster-burning powders. Pressure needs to develop quickly to get maximum benefit before the bullet leaves the barrel. At the other end of the spectrum, larger rifle calibers tend to use slower-burning powders. They are designed to keep burning and creating expanding gas while the bullet travels down the longer barrel. Since the burn rate is slower, pressure doesn’t spike as high at the peak, but it develops over a longer period of time.
The trick to getting safe, good performance out of any caliber reload is making sure the pressure curve is right for the job. The burn rate of the powder (and rate at which it creates expanding gas) has to be in harmony with the volume of the cartridge, barrel length, projectile weight, and designed strength of the gun. Fortunately, you don’t have to worry about this if you stick to published load recipes in reloading manuals.
Since cartridges are sensitive to very small fluctuations in powder weight, a different measurement unit is used: grains. A grain sounds like it might refer to a “piece” of powder, but it’s just a weight measurement—one seven-thousandths of a pound, to be exact.
It’s a really big deal to get the weight of the powder charge exactly right. Too little powder might not generate enough pressure to shove the bullet all the way out of the barrel. Another shot behind one that lodges a bullet in the barrel can blow up your gun. Too much powder can generate too much of a pressure spike, which can also blow up your gun. These are both less than ideal scenarios.
The Lyman powder dispenser shown here is a common design for a powder dispenser. You fill a reservoir (that’s a fancy name for the big plastic hopper) with your powder of choice. The reservoir feeds into a handle-operated dispenser. Within the dispenser body is a cleverly-designed cylindrical cavity. This cavity features a movable “plug” that’s called a metering bar. It’s made of three interconnected bars: a large segment, a medium one, and a small one. They are independently controlled by finger screws and together determine how much of the cavity is taken up by the plug. The open space remaining determines how much powder is allowed to drop into the cavity from the reservoir each time the handle is operated. By sliding the three metering bars—large to get close, medium to get closer, and small to get fine volume measurement—you can size the cavity to drop the exact powder charge you want, down to the tenth of a grain.
Adjusting the cavity with the metering bars is a trial and error method. While you’ll be able to eyeball your way close after a couple of reloading sessions, the first time will take a few tries. If you’re reloading a pistol cartridge, you can probably start with the large bar closed. Open the medium bar a bit, drop a charge into a cartridge casing, and pour the powder into your scale pan. When you see the actual weight, you’ll know whether to adjust your metering bars more open or closed. Repeat the process until it’s set to drop the exact weight you want. Most powder dispensers have a finger screw to “lock” the metering bars in place once you get them set right, so be sure to fasten that down too.
Powder dispensers have a way of “settling” into a groove, so be sure to drop a number of charges, weighing each one, before you start filling up lots of cases. You may find you have to readjust your measuring bars as the powder settles. I find it also helps to keep your powder reservoir fairly full. If you start full and let it get to almost empty, you may find that your charges are inconsistent as the powder level goes down.
After the powder drops in, I take a look in the case to make sure it looks about right and then place the “charged” case into a reloading tray. Every five or 10 cases, I’ll dump the powder contents into the scale to make sure that my dispenser is still dropping the right amount of powder.
When the tray is full, usually with 50 charged cartridge cases, I hold it up to a light and scan down the rows of cartridge cases to visually verify that all the charges look the same. Consider this another safety check to make sure none of the cases have no charge or an overcharge.
This method is perfectly acceptable for volume reloading of rifle and pistol cartridges. A good powder dispenser will get you consistently within one tenth of a grain of the desired powder charge weight. If you want super-accurate charges for super-consistent shooting, you can always weigh each powder charge individually using a combo dispenser and scale, but that’s a topic for another day.
Next up, the last steps in the reloading process: seating, crimping, and inspecting.
Tom McHale is the author of the Insanely Practical Guides book series that guides new and experienced shooters alike in a fun, approachable, and practical way. His books are available in print and eBook format on Amazon.
