IV. PowdersA. IntroductionSmokeless powder has several characteristics that affect performance. The two most significant are density and burn rate.
Powder density has two separate meanings to the reloader. Volumetric density is an expression coined by Richard Lee, because ?density? can be somewhat ambiguous. Load density refers to how much powder is in the case. Volumetric density is the volume displaced by one grain of a particular powder, or the density of that particular powder. Both are critical, which will be explained below.
Burn rate refers to how fast a particular powder combusts into propellant gases. In general, fast-burning powders are better suited to low pressure loadings. Slower-burning powders give higher velocities and more consistent results, especially in high pressure loadings. Essentially, a slower burning powder also builds pressure more slowly, which keeps the peak pressure lower. Slower powders tend to yield maximum load density. In handgun cartridges, powder burn rate seems to be the single most important characteristic for top accuracy, assuming correct loading techniques and suitable cases.
B. Powder Density and VolumeThe most important property of either blackpowder or smokeless powder is that it also occupies volume in the cartridge case. In other words, one can think of powder in terms of charge weight and/or volume. Charge weight is a convenient way to measure powder, and it is usually the most accurate with current equipment. However, thinking solely in terms of powder weight will limit your results. Consider the fact that all powder measures and similar equipment dispense powder by volume, not weight.
The relationship between available case volume and the volume of the powder is known as load density. A high load density means that there is little or no airspace in the case. A low load density means that the powder occupies less of the case volume, resulting in some airspace. Slower burning powders tend to yield higher load density, because they require heavier charge weights and are ?bulky? (lower volumetric density). In other words, the charge weight is heavier (more grains), and each grain also takes up more space as compared to most fast-burning powders.
It is very important that the reloader understands the relationship between mass and volume. As the charge weight increases, the load density increases, so there is less airspace. Either factor alone would raise pressure significantly. Obviously, more powder generates more energy, raising pressure. Less obvious is that more powder occupies more space, which also raises pressure, as the expanding gases have less available volume. Certain fast-burning powders can cause dangerous pressures with extremely small increases in charge weight.
9mm Luger was designed for use with smokeless powder. In practical terms, this means that most powders intended for handgun use will work in 9mm and yield decent load density. For purposes of comparison, let?s look at .38 Special, which was designed for use with blackpowder. Blackpowder has a low volumetric density (a grain takes up more space) and a high charge weight, as compared to the more efficient smokeless powders. This means that .38 Special and similar cartridges (.44 Special, .45 Colt, etc.) have too large a case volume for many smokeless powders. .38 Special is also low in pressure, which means that a faster-burning powder will be required. Faster powders use lower charge weights and yield a low load density. All this adds up to a problem. Typical .38 Special charges are less than 4 grains of powder, creating far too much airspace. Low load density of this type can cause major problems. If the powder is strung out through the large case, it may not all ignite at the same time or at all. If it does, it will burn unevenly, or more like a fuse. Powder may not even contact the primer at all.
Again, this is never a major concern with 9mm, as the case volume is so small, less than 0.7 cc (cubic centimeters). Fortunately, this also means that it is fairly easy to find an optimal load. A higher load density, or even slight compression, tends to yield better accuracy with extruded powders. The powder position issues discussed above are not a factor. Since 9mm is a fairly high pressure cartridge, or 35,000 psi max SAAMI standard, it is better suited to medium to slow burning powders. This means that maximum load density (little or no airspace, no compression) is obtainable with most powders commonly used in 9mm. The only downside is that 9mm is that has so little volume that it becomes potentially dangerous with even small (0.1 gr) increases in charge weight with the faster-burning powders.
Most other handgun cartridges do not fall victim to these extremes. Since .40 S&W is essentially a reduced volume version of 10mm, it should probably be considered in the same category as 9mm. Avoid faster-burning powders that do not overflow the case with a double charge.
Nobel-Vectan ball powders must never be compressed. There must be some airspace in the cartridge case even if it is only the infinitesimally small spaces between grains of powder. Nobel-Vectan specifically warns against compressing their ball powders, as their smaller spheres will pack too densely. Instead of igniting consistently, the primer drives the charge like a piston, which can create dangerously high pressures. 9mm data using ball powder will sometimes seem artificially conservative; this is because some publishers also do not list compressed loads. In particular, I have noticed that Winchester itself no longer lists compressed loads with W-231. Extruded and flake powders can be compressed, and accuracy may even improve with slight compression. The shape of the powder prevents it from packing tightly into the case.
C. Powder Burn RatesBefore proceeding, it may be helpful to consult the following link:
"
http://www.reloadbench.com/burn.html"
This webpage provides some definitions and a burn rate chart to which I will refer.
The Norma Reloading Manual contains an article on smokeless powders written by Sven-Eric Johansson of Nexplo/Bofors. I mention this because it is by far the best treatment of the subject I have ever seen; it even includes photographs detailing every step of the manufacturing processes.
Mr. Johansson defines burn rate as follows:
?The linear burning rate of a propellant is the rate at which the chemical reaction progresses via both thermal conduction and radiation. The burning rate is equivalent to the distance (normal to the burning surface of a powder grain) burned through in a unit of time. This varies not only with composition, pressure, temperature, and physical structure of the powder (i.e. porosity, density), but also with the shape of the powder grain. Powder with a high calorific value burns faster than powder with a low calorific value? (102).
Due to their higher calorific value, fast-burning powders also tend to produce more heat as a form of wasted energy. There is a direct geometrical progression between maximum chamber temperature and burn rate as the burn rate increases (becomes ?faster?). Environmental factors such as ambient air temperature and relative humidity also affect burn rate. Ammunition generates higher peak pressures on hot, dry days by increasing the powder?s burning rate. Leaving loaded cartridges in direct sunlight has a similar effect.
In ballistic terms, the slowest practicable powder will generate more uniform internal ballistics, lower chamber pressures, lower chamber temperatures, and higher velocities. Slower powders require heavier charge weights and often have a low volumetric density. To clarify, this means that the heavier charge is also relatively bulky, because one grain of the powder also takes up more space. Some powders are too slow-burning for some handgun cartridges, because the case cannot hold enough of that particular powder or it will not burn completely.
The working pressure of the cartridge is the most important factor in selecting the correct burn rate. A higher peak pressure equates to a slower burn rate. 9mm has a relatively high peak pressure for a handgun cartridge: 35,000 psi SAAMI and 38,500 psi SAAMI +P. This means that the slower-burning powder can combust more slowly and build pressure in a slow, uniform, geometric progression. This allows for more gradual and complete acceleration at a lower relative pressure and temperature. Conversely, a low pressure cartridge like .38 Special requires a faster-burning powder with a higher calorific value. Slower powders will not ignite and combust evenly at such a low working pressure.
By consulting the burn rate chart at "
http://www.reloadbench.com/burn.html" and comparing it to your reloading manuals, you will make some immediate observations. Although you will find published loads using Norma R-1 (#1, the fastest-burning powder) to Alliant 2400 (#67, extremely slow for handgun applications), the vast majority of handgun cartridges use the powders in the #6 to #63 range, or Alliant Bullseye to Accurate Arms No. 9.
These statements are all generally true, but some powders have peculiar characteristics. For example, Alliant Unique generates relatively high chamber temperatures for its burn rate. Universal Clays, which is theoretically identical to Unique, does not generate as much heat, but tends to generate a higher peak pressure. Unfortunately, this can be load or cartridge specific, and you will have to learn by experience which powders will work best in your particular pistol. However, this is half the fun as well.
In summary, best accuracy for handgun loads are usually obtained by using one of the slowest applicable powders that allows for a high load density. With a Nobel-Vectan ball powder or any powder with exceptionally small grains or spheres, some airspace is desirable for safety reasons. With an extruded powder, maximum load density (little or no airspace) or even slight compression usually yields the best accuracy at a velocity higher than most factory loadings. This is due to the fact that powder position has been completely removed from the equation.
D. Other ConsiderationsThe above section is an oversimplification, because one cannot select a powder based on burn rate and load density alone. Powder manufacturers deliberately alter the characteristics of powders by adding deterrent coatings, decreasing volumetric density with cellulose, etc. to provide the reloader with more options. A brief discussion of some of the other considerations follows.
Availability is obvious, but often ignored by some well-meaning folks. A casual glance at the burning rate chart reveals that several of the powders suitable for handgun use are manufactured by Norma (Sweden), VihtaVouri (Finland), and Nobel-Vectan (France). Odds are, you may not be able to walk into the corner gun store and buy these in 1 lb. canisters. As of 2006, the Nobel-Vectan powders were no longer imported into the U.S.
Economy is a major factor for many. Slower powders tend to use heavier charge weights, and powder is sold by weight, not volume. Some of the slower handgun powders, such as Alliant Blue Dot, also cost more by the pound than other powders.
Metering is a major concern for most handgun reloaders, because they shoot so many cartridges in a month. Metering refers to how well equipment can uniformly dispense the powder. Ball powders meter extremely well, because the spheres are so small and uniform. Older extruded and flake powders do not usually meter well.
Cleanliness is a major concern for some shooters; some could not care less. Generally, selecting the correct burn rate will equate to a complete and clean burn. Some older powders, such as Unique, are relatively sooty despite this fact.
Powder selection is somewhat dependent on the intended use. If you want to load high performance ammunition, you will need to use a slower-burning powder, especially in the magnum revolver cartridges or cartridges with limited volume, such as 9mm and .40S&W. If your goal is to assemble accurate ammunition of a particular velocity (i.e. to make Minor PF) and do so economically, a faster powder with a higher calorific value and lower charge weight may be adequate.
When compressing an extruded or flake powder, you are essentially wasting some powder to gain uniformity. The minute cartridge-to-cartridge variations do not factor into the equation, because only X amount of powder ignites. Similarly, you have eliminated the variable of powder position relative to the primer. Some of the older powders that are less popular today, like Alliant Herco, can be surprisingly accurate.
F. Summary1. Smokeless powders have several characteristics that influence performance. The most important are powder density and burn rate.
2. Faster burning powders are more efficient for low pressure handgun cartridges, but their use limits potential velocity.
3. Higher load density usually gives better results in handgun cartridges.
4. A burn rate chart may be helpful in selecting a powder, but it does not provide the entire picture, as factors such as availability, economy, metering, and the like should be considered as well.
As authored by Radom, June 2010
