Ammunition Basics

Introduction

In this lecture we’ll take a look at ammunition. We’ll cover the basic composition of a “bullet” and how that relates to wounding. We’ll also take a look at some interesting types of ammunition that may be encountered in the United States. First, let’s talk a little about the basic unit of ammunition, the cartridge.

The cartridge

Notice above I put the word “bullet” in quotation marks. That’s because the bullet is only one part of the whole round, or cartridge. Please forgive my meager Paint skills and take a look at the diagram below:

The “bullet” is just the piece of the whole setup that exits the firearm. The entire thing is referred to as a “cartridge.” A metal (most commonly brass) casing holds all the pieces together. At the base of the cartridge is the primer. This contains a small amount of combustible material that will ignite the propellant. Combustion of the propellant then forces the bullet away from the cartridge and down the barrel of the firearm. Since the casing is all one piece, it acts to prevent any rearward or sideward escape of gas, which would reduce the efficiency of the reaction. 

Another part of the cartridge that you can’t see above is the head stamp, which might look something like this (again, forgive the bad image skills)

Most cartridges have head stamps on the bottom that usually include some type of branding and the size of the projectile. The round circle in the center is where the hammer strikes, which initiates the combustion process. If you’ll recall from the last lecture, this is one of the identifying features that’s used by NIBIN. 

There is a lot of variation to rounds, but this is the basic structure. Rifle ammunition doesn’t differ significantly from handgun ammunition. It is often assumed that rifle ammunition is, by definition, larger than handgun ammunition. This can be true, but the most common rifle ammunition that is encountered in the US is actually a .22 caliber rimfire round:

Rimfire just means that the primer is located in the flange at the base of the round that is slightly wider than the cartridge itself. This is in contrast to the above diagram, which would be referred to as “center fire.” Like most of the stuff we’ve talked about, this doesn’t necessarily alter the wounding pattern of the round itself. One important thing to notice, however, is that the casing is much larger, which means that it holds a whole lot more primer. THIS will impact wounding in a way we’ll cover later. Another type of rifle round that you may see once in a while in the US is the 7.62 round. These are typically used in high power rifles like the AK-47.

This is a large round. It’s probably the largest one you will encounter when approaching gunshot wound cases. Now, let’s move onto something really confusing.

Ammunition measurements

In the previous section, we threw out a couple of numbers that both refer to sizes of bullets. The two most common ways that one refers to bullets are a number that refers to a measurement of the size of the cartridge or the caliber. Size measurements are pretty self-explanatory. For example, a 9 mm cartridge is approximately 9 mm from base to tip. The aforemention 7.62 round is 7.62 cm from base to tip. Caliber measurements are a little different.

Remember when we talked about rifled barrels? To review, lands are the things that protrude into the barrel and grooves are the spaces between. Caliber is the distance between two opposite lands. It also refers to the bore or the diameter of the barrel. Unlike plain size measurements, caliber measurements are given in inches. So a .45 caliber round is one that fits a barrel that is .45 inches from land to land. I say it like this, because in reality, .45 caliber cartridges don’t have to fit that exact measurement as one gun is usually capable of chambering and firing bullets of several different caliber classifications. In general, these designations are inconsistent.

A final measurement that is used when describing ammunition is the weight of the projectile. As a forensic pathologist, you will be weighing the bullet rather than the whole cartridge. Pathologists usually weigh things in the metric system (i.e. milligrams, grams, kilograms) but projectiles are usually measured in grains (gr). One grain is about 65 milligrams or 0.002 ounces. A grain is a pretty fine measurement and usually rounds of different sizes only differ in weight by a minuscule amount. 

So why are these inconsistently used measurements important? Oftentimes, more than one type of weapon is used in an event. By comparing the measurements of the projectiles and fragments that we find during the autopsy to the casings and/or firearms found at the scene, a better picture of the event can be assembled. Recall that we are in the business of pattern recognition. We can say that a particular finding is consistent with a specific event, but we can’t definitively say that a finding is due to a specific event. 

Now let’s focus on a specific part of the cartridge: the bullet.

The Bullet

The bullet is the thing at the end of the cartridge that exits the barrel of the gun. Most modern bullets consist of a metal core (can be lead, copper-tin alloy, tungsten polymer, etc) that may or may not be surrounded by a metal (usually copper) jacket. The tip of the bullet may be rounded or have a depression in it that is referred to as a hollow-point. Below are some examples. These are bullets procured during autopsies, so they will be deformed in one way or another.

Finding whole, intact bullets during autopsies is not super common. Bullets are usually made of softer metals that tend to fragment or deform when they strike targets. In fact, they may  be designed to do this. Bullets of different construction have different effects and may be marketed in ways that reflect this. For example, hollow-point ammunition, or ammunition that is otherwise easy to deform may be marketed as “home defense” or “low penetration” rounds.

The reason for this has to do with the dynamics of the projectile. A pointed, full metal jacket bullet is less likely to deform when it strikes a target. It therefore tends to lose less energy to that target and may travel a bit further. On the other hand, a hollow-point bullet tends to deform quite easily when it strikes a target, which means that its energy is more easily transferred to its surroundings, which tends to prevent it from traveling very far after impact. Here’s an example of this type of round doing what it was designed to do:

Note the mushrooming of the projectile. The pink object below the projectile is a fragment of polymer that is sometimes embedded in the tip of a hollow-point round that is supposed to aid the deformation of the bullet. It should be easy to see that this projectile is much less aerodynamic than the fully jacketed ones above. This will impact tissue damage in ways that we’ll cover in the next section.

That about covers the basics of ammunition for rifled barrels. Before we move on to shotgun ammunition, let’s take a look at some special types of handgun ammunition.

Magnum Rounds

A magnum round is just one that has an elongated cartridge that holds more propellant. The purpose of this is to increase the muzzle velocity of the bullet.

Sabot Rounds

A sabot is a structure that encases a bullet and is designed to fall away from it after it leaves the barrel of the gun. In the modern day, sabots are usually plastic. This type of ammunition is pretty uncommon. I couldn’t find any public domain images to use, so my crude paint diagram will have to suffice:

The purpose of these rounds is to increase the muzzle velocity by reducing friction between the barrel and the bullet. There are three ways that a sabot could could affect your autopsy. First, if the round is fired close enough to an individual, pieces of the sabot may strike or become embedded in the skin, causing a patterned injury. Second, the bullet itself will have no rifling on it as this will only be imparted on the sabot. Bullets without rifling are not commonly seen, so this would definitely stick out. Finally, any unexplained bits of plastic found on or near a body may represent a fragmented sabot. These findings could influence the investigation.

Glaser Rounds

This type of round consists of a copper jacket that surrounds multiple small pellets. The round is capped by a polymer plug similar to the hollow-point we saw above:

As you might imagine, this is going to look totally different than a normal gunshot wound. It will look more like a shotgun injury. It’s good to have an awareness of out of the ordinary stuff like this.

Frangible Ammunition

These are similar to Glaser rounds in that they consist of a hollow bullet that is filled with, in this case, copper powder. They are designed to disintegrate when they strike a target, limiting penetration and reducing the possibility of ricochet.  

Black Talon

This is a proprietary round made by Winchester:

As you can see, it is a modified hollow-point round that has six barbs that fold back when it strikes a target. These haven’t actually been sold since 1993, but they are still out there (just search “black talon” on Google and see what the suggested auto-completes are). Most recently, these were in the news as they were the type of ammunition that was used in the shooting death of Reeva Steenkamp in South Africa. Why should you be aware of them? Most practically, the barbs are extremely sharp and can easily pierce surgical gloves. So how would you know you are dealing with one of these? Luckily, they have a pretty distinct x-ray shadow that sort of looks like someone putting their hands on their hips:

I’ve only dealt with this type of ammunition once before. It has more of a tendency to show up on the forensic pathology board examination than in real life. 

RIP ammunition

This is a newer type of round that consists of several metal flechettes instead of an actual bullet. RIP stands for “radically invasive projectile.”

I’ve had a couple of cases involving these. The flechettes separate from the cartridge and create a multitude of small, triangular defects. They seem able to penetrate into things pretty well. They are also fairly expensive, so maybe that accounts for their low prevalence in gunshot wound cases.

Now let’s take a look at shotgun ammunition.

Shotgun Ammunition

Shotgun ammunition is fundamentally different than handgun ammunition. This ammunition usually consists of a collection of spherical metal projectiles packed into a plastic cylinder that also contains propellant, and a paper or plastic wad that occupies space that’s not filled with propellant:

When the shotgun is fired, the shell breaks apart, sending the shot out of the barrel in a clump that spreads out as the distance between the barrel and the pellets increases. 

Like with handgun ammunition, conventions for classifying shotgun ammunition are confusing and inconsistent. Something you hear a lot when referring to shotguns is the “gauge” of the shotgun. This is actually an archaic term that refers to the number of lead balls of a certain diameter (bore) that weigh one pound. I don’t think this term has any relation to modern shotguns or shotgun ammunition. More commonly, shotgun ammunition is divided into birdshot and buckshot, depending on the size of the pellets.

It shouldn’t be hard to assume that birdshot pellets are going to be smaller than buckshot pellets. Shot comes in varying sizes that are usually denoted by a number. There are tables out there that denote the size of the pellets relative to the shot classification, but it’s beyond what we’re interested in here. It’s more important to note the different parts of the shotgun shell as those are all moving objects that could strike a person and alter wounding.

So now that we have the basics of both guns and ammunition down, we’re going to put the two together and see what happens when a gun is fired and when a bullet strikes a person.