Most reloaders understand that excellent accuracy can be achieved if components are of good quality, consistent dimension and properly assembled; but the wrong amount or type of powder may not produce accurate results no matter how much care has been taken to make each load. Here is a little theory and an old technique that may help improve the accuracy of your loads.
Powders with similar burn rates (quickness) as listed in a reloading table often will not deliver similar results. There is a simple reason for this. The burn rate is only one powder characteristic and may not be the primary factor contributing to accuracy. Some purists prefer to test every powder at various weights but this can be costly and time consuming. There is an old time-tested way to select the first powders to test.
Ballisticians have long recognized that the volume of powder in a cartridge effects the accuracy of a load. A powder charge that completely fills the space under the bullet is often more accurate. It is theorized that this is due to more consistent ignition and rise in pressure behind the bullet, resulting in more consistent velocities and therefore greater accuracy.
The percent of the available powder space filled by the powder is called the "load density". As the load density is reduced (powder doesn't fill case), the point of peak pressure moves toward the muzzle and velocity falls off. With a partially filled case the powder charge can shift to change ignition characteristics, the speed of burn, pressure curve and resulting velocity. In contrast, increasing the load density moves the point of peak pressure backward toward the chamber and increases velocity. Since the powder cannot shift within the cartridge, ignition and pressure is more uniform, resulting in more consistent velocities and better accuracy.
For most loads the powder has completely burned by the time the bullet travels two or three inches beyond the point of peak pressure, but if the firearm has a short barrel and low load density the peak pressure may move toward the muzzle resulting in excessive "muzzle flash". Excessive muzzle flash happens when unburned powder ignites at the muzzle, causing turbulence behind the bullet and a detrimental effect on accuracy. Ideally, the peak pressure should be developed near the chamber for a complete burn before the bullet exits.
There is usually a direct relationship between the burn rate (quickness of the powder) and the space a powder will fill. Powders may be made to burn quicker by reducing the granule size, which increases its density so a given charge weight takes up less space. Simply switching to a quicker powder may not resolve a muzzle flash problem if the powder has a high density and doesn't fill the case. Instead, filling the case completely with a lower density, but slower powder (as listed in a load table), may actually hold the point of peak pressure nearer the chamber for a complete burn, less muzzle flash and better accuracy.
Reloaders interested in accuracy have loaded cartridges originally designed for black powder with smokeless powder and everything from cereal to cardboard wads in an effort to fill the excess space. The safety of this practice is questionable and should NEVER be attempted without first consulting a reliable load reference, but those who use fillers swear by the results. This practice is testament to the validity of load density theory.
If you do not own software that will calculate load densities there is a manual method that, although time consuming, will work. First it is necessary to determine the powder capacity of the cartridge. To do this, file a small notch inside the mouth of a fired but not decapped case so it extends from the rim of the case to a point just beneath the bullet's base when it is seated. Weigh the case and bullet. Fill the case with water and carefully seat the bullet while displacing water out the notch filed in the mouth. Weigh the water filled case and bullet again. The difference in weights is the powder capacity of the cartridge in grains of water.
Next, you must know the bulk density of the powder you are using. Some powder companies publish bulk densities, others force you to find the bulk density yourself. The bulk density of powder is most commonly expressed as grams per cubic centimeter (g/cc). Since the weight of one cc of water is nearly always 1 gram, a bulk density of 0.970 g/cc means the powder is 97% as dense as water. Bulk density is therefore also the powder's weight in any volume divided by the weight of water filling the same volume. To find the bulk density for a powder first weigh a container (large as practical for your reloading scale) to determine it's weight. Then weigh it filled to the top with powder. Fill the empty container with water and weigh it again. Divide the weight of powder the container will hold by the weight of water it will hold. The result (1.000 or less) is the bulk density of the powder.
To finish the calculation use these formulae:
Cartridge Cap. in gr. of H20 * Powder's Bulk Density = Powder Capacity
Powder Charge in gr. / Cartridge's Powder Capacity = Load Density
Most powder companies will admit that the bulk density of production lots differ by as much as ±10%. You should consider this limitation when comparing load densities. Powders that produce load densities within 10% of each other may provide similar results and should not be eliminated from consideration. All rules are made to be broken. The theory of load density related to accuracy is not entirely foolproof, particularly when applied to small cartridges or certain actions. Any safe charge producing a load density of 80% or more may be worth testing if powders with higher densities do not produce the desired result.
A chronograph can help validate a particular powder charge. Once you have narrowed the list of powder options, load at least 10 rounds of each powder charge and test fire them at rest through a chronograph. Shoot each test load at different targets. If you make any bad shots, carefully mark them so human error can be considered. You should find a direct correlation between the group size for each target and the measured velocity for each load. Load tests with the highest velocity variation should have the largest group size and tests with the lowest velocity variation should be more accurate.
If the velocities achieved with a load are consistent and muzzle flash is minimal, it should be a good load. Primer and powder weight irregularities can typically be eliminated as the source of accuracy problems if consistent velocities are produced. If test velocities are consistent but accuracy is still poor, then look for bullet/seating irregularities or a mechanical problem with the firearm.