Shotgun Pattern Testing

Another test conducted by firearm examiners is known as Shotgun Pattern Testing.  This test involves shotguns and allows for a muzzle-to-target distance to be determined.

Shotgun pattern testing involves examining evidence for a pattern of holes created by the pellets fired from a shotgun.  The "unknown" pattern is then compared to "test" patterns created with the suspect shotgun fired at known distances.  This will allow for an approximate muzzle-to-target distance to be determined.

Two overlapping shot patterns.  A large dispersed 
pattern overlaps a small close-range shot pattern.

When a shotgun is fired using a multiple pellet shotshell, the pellets exit the barrel of the shotgun and begin to spread out into a pattern that increases in diameter as the distance increases between the pellets and the shotgun.

To better understand the principles involved in shotgun pattern testing it's important to first learn a little about shotguns and the shotshells they fire.

Shotguns are firearms typically fired from the shoulder that are designed to fire shotshells containing anywhere from one large projectile to as many as several hundred small pellets.  Shotguns aren't classified by caliber but come in different gaugesThe gauge of a shotgun is determined by the number of round lead balls of bore diameter that it takes to equal one pound.  Shotguns can come in 10, 12, 16, 20, 28, and .410 gauge.  The .410 is actually an exception with .410 referring to the caliber of the shotgun's bore.  It would actually be about a 67 gauge in "lead ball" terms.

Although some newer shotgun barrels are produced with rifling, shotguns have traditionally had smooth bored barrels.  Except in some rare cases the projectiles fired from them cannot be matched back to the shotgun.

Auto loading shotgun with conventional barrel (top) and 
an auto loading shotgun with a rifled "slug" barrel (bottom).

Shotguns come in a number of different styles and actions.  From auto loading shotguns like those seen above to very "customized" versions like the one below.

"Sawed-off" pump-action shotgun.

Shotguns are typically manufactured with what is called a choke in their barrels.  A choke is a constriction in the last couple of inches in the barrel and can vary in the degree of constriction.  Common choke designations are "full", "modified", and "improved cylinder."  A barrel that has no choke is referred to as a cylinder-bore barrel.

Barrel modified for use in a "Turkey-Shoot" competition.
Extreme constriction or choke can be seen at the muzzle.

Shotguns can be manufactured with a permanent fixed choke or can have the muzzle of the barrel machined in a way to accept interchangeable or adjustable choke tubes.  Shotguns that have had their barrels sawed off have had their choke removed.  This creates a shotgun with a cylinder-bore barrel.

The whole point to this "choke thing" is that the choke plays an important role in the rate at which the shot pellets spread as they travel away from the shotgun.  A full-choke barrel will tend to shoot smaller shot patterns at a given distance than a barrel with a modified-choke.

Shotshells are cartridges designed to be fired in shotguns and can contain a single large projectile - a slug - or as many as several hundred small spherical pellets called shot.  Shot used in shotshells has traditionally been made of lead but  because of it's toxicity, other materials are being used as a substitute, with the most common alternative being steel.

The size of the shot can vary as can the total weight of the shot loaded into a shotshell.  Shot comes in two basic varieties, small pellets commonly referred to as birdshot and larger pellets called buckshot.

Components from a typical shotshell containing 
birdshot and a one-piece plastic wad.

Components from a typical shotshell containing 
buckshot and a fiber/plastic wad combination.

Lead birdshot comes in 12, 11, 9, 8 1/2, 8, 7 1/2, 6, 5, 4, 2, and BB sizes.  As the numbers get smaller the diameter of the shot gets larger.  Buckshot on the other had comes in 4, 3, 2, 1, 0, 00, and 000.  Again, as the number decreases the diameter increases.  See the chart below.

Shot size table: lead shot (top), steel shot 
(middle) and buckshot (bottom).

As you can see from the above chart, steel shot comes in slightly larger sizes than lead shot.  Steel doesn't have the density of lead and larger shot is needed to achieve a range comparable to that of lead shot.

Shotshells contain a variety of different wads - plastic, paper, or fiber material designed to separate the shot from the gunpowder and/or protect the shot as it is pushed down the barrel - that are expelled from the shotshell, along with the shot, when fired.

Various plastic and fiber shotshell wads.

Shotshells come in a variety of loads.  The amount of gunpowder in a shotshell can vary and the measurement is referred to by as the dram equivalent.  The dram equivalent is the amount of smokeless powder that produces a velocity comparable to that of black powder.

All of these variables are important in determining a given shot pattern distance.

When a shotgun is fired the shot and wadding travel down the barrel and exit the muzzle in a concentrated mass.

As a result a contact entrance hole will produce a large hole with significant damage to the margins of the hole, but can vary greatly depending on the material being fired into.  The same thing also applies to gunshot residue deposits.  Most contact entrance holes will have a significant deposit of gunshot residues like the one seen below, but this is not always the case.  Some may display very little visible gunshot residue.

Contact shotgun entrance hole.

A hole like the one above will be processed chemically like that previously described on the Distance Determination/Gunshot Residue pages.

At ranges of around 5-10 feet* the shot and wadding mass will produce a single large hole in a target.  If the target happens to be a person, the wadding material will be blown into the wound tract with the pellets.

Close-range shotshell pellet entrance hole.

The close-range entrance (less than 5 feet*)hole seen above is almost square, and is a common shape for this range.  You might notice a pinkish color (lead residue) to the material around the hole.

At distances greater than 5-10 feet* the shot mass starts to break up.  Fliers (individual pellet holes) will start to appear around the edge of an entrance hole and the wadding may or may not enter the victim.

Individual pellets starting to break apart from the main mass of pellets.

As the wadding slows down it will start to take a separate trajectory from that of the shot and can actually leave abrasions or bruises to the area around an entrance wound.  Wadding will lose its energy and fall harmlessly to the ground at distances of around 20 feet*.

As the pellets get further and further away from the shotgun the pattern will eventually become dispersed to the point that only individual pellet holes are present in a target.

Witness panel fired into at a distance of 28 feet.

Firearm examiners will try to reproduce the pattern by firing into witness panels at known distances.  Shot patterns can be affected by the load, pellet size, wad type, and choke of the shotgun.  That is why it is essential that the shotgun is recovered and the type of shotshells used is known.  Hopefully some shotshells will be recovered at the scene that can later be used in firing the distance standards.  Also, patterns produced by a shotgun at any given distance can vary slightly.  Multiple tests patterns will be fired at known distances and compared directly to the pattern in question.  Based on this comparison a minimum and maximum firing distance can be determined.

Unlike the tests conducted on clothing for gunshot residues, shotgun pattern testing is not limited to distances of a few feet or less.

*All distances are approximate values and can vary depending on the shotgun's gauge/choke and ammunition used.


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