Ball Powder Goes the Distance

Effect of gravity and air friction on gunpowder

Effect of gravity and air friction on gunpowder

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Ultimate fate of gunpowder particles in flight their cartridges. This pretty well ensures the fact that unburned and partially burned gunpowder particles will exit the barrel of a weapon when fired.

Regardless of how much or what type of gunpowder is involved, the force of gravity and the effect of air friction both act to slow and eventually bring the powder particles to a halt. Thus, if a weapon is held parallel to the ground and fired, the powder will only go a limited distance before it lands on the ground. Exactly how far this distance might be depends, once again, on things like barrel length and cartridge composition (shape and amount of powder and bullet weight).

In attempting to determine the approximate muzzle-to-target distance, the firearms examiner may also use the behavior of soot and vaporous lead (coming mostly from the lead azide primer). These materials tend to be deposited at very close distances, less than about a foot. When the deposits are present, the examiner can often further define the estimated distance.

The whole process begins with visual and microscopic examination (by stereoscope) to locate the powder particles, followed by chemical testing for lead and nitrites using the sodium rhodizonate test and the Griess test, respectively. Nitrites are the product of the combustion of nitrates, the chemical entity of gunpowder. Once a pattern of nitrites and/or confirmation of vaporous lead has been established, the firearms examiner then carries out a series of test firings using the weapon and ammunition like that used in the shooting. When a similar pattern has been produced and is found to be reproducible, the firearms examiner is able to propose an approximate muzzle-to-target distance.

In the case of a shotgun firing shot, the diameter or density of the shot pattern produced can also be used to help establish distance. Just as gunpowder particles, steel, or lead shot exiting the muzzle of a shotgun travels in a conical distribution, the base diameter of the pattern becomes greater the farther from the muzzle it is. Also like gunpowder particles, the shot is acted upon by the effects of gravity and air friction, which tend to bring the shot pellets to the ground. A rough approximation that works best with buckshot loads is that for each inch of shot pattern diameter, one has another yard of muzzle to target distance. To record the shot pattern a firearms examiner draws a circle that includes all of the shot holes, then uses the diameter of the circle to approximate the distance between the muzzle and the target, as illustrated in the diagram. The results are then confirmed through test firing.

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