Firearm Basics

Introduction

This will be the second stop along the way in our discussion about gunshot wounds. This will be a pretty short introduction to the common types of firearms that are used in the United States. I am not a firearms or ballistics expert, so what we’ll be talking about here will be pretty basic. However, I think this is important to cover as there are some things that we take into consideration when we approach gunshot wound cases. A basic knowledge about the type of firearm used can help align autopsy findings and circumstantial information. As we’ll see later, gunshot wounds are incredibly varied and having more information always helps makes sense of them. Let’s start by taking a look at a very broad classification of firearms related to characteristics of their barrels.

Rifled vs. Non-rifled 

Rifling refers to a pattern of grooves in the barrel of the gun that impart a spin to a fired projectile, thus stabilizing its flight path:

Now that’s the barrel of a tank gun, but the idea is the same on a smaller scale. Rifled barrels are found in handguns, rifles, submachine guns, and machine guns. 

Rifling divides the barrel into two sections: lands that protrude into the lumen of the barrel and grooves between them. Most importantly, the rifling imparts a pattern onto the soft metal of the projectile or projectile jacket as it travels down the barrel:

This is an important feature because it’s one way that projectiles can be traced back to the gun that fired them. The combination of rifling pattern and the imprint of a firearm’s firing pin on the base of a bullet are considered fairly specific for a particular firearm. In other words, high-quality, standardized imaging studies can compare spent casings and projectiles in order to identify weapons used in shooting events. A large national database called the National Integrated Ballistic Information Network (NIBIN) exists for this very reason. It’s a pretty interesting tool that crime labs can use. You can read up on NIBIN here. 

From here, let’s take a look at two of the examples of rifled firearms we mentioned above: handguns and rifles.

Handguns

Handguns can be most easily divided into revolvers and auto-loading pistols. There are a few other, less common types such as derringers, specialty single shot pistols, and fully automatic pistols that aren’t worth mentioning as they do not differ mechanically in any significant way. Let’s start with revolvers.

Revolvers

Revolvers are mechanically simple handguns that consist of a rotating cylinder which contains multiple chambers. The most common type of revolver has a cylinder that swings outward for loading. When the trigger is pulled, the hammer is propelled forward, striking the firing cap of the loaded cartridge, which fires the gun. Some revolvers need to be manually cocked (the hammer must be pulled back into the firing position) with each shot. Others have mechanisms that pull back the hammer and cock the gun with each trigger pull. The cartridge is not ejected from the cylinder upon firing. One thing that is important to note is that revolvers do not have the same safety features that autoloading pistols do.

From a forensic pathology standpoint, there are a couple of things that are useful to know about revolvers. Due to their their open cylinders, they tend to eject more material into the surrounding air. This basically includes gas produced from burned propellant in the gun. If it is an excessive amount, it can sometimes stain the hand of the user. It can also affect the appearance of the wound itself. Soot staining and stippling of skin is something we’ll talk about a little later. Let’s move onto our next category.

Autoloading Pistols

A quick word on terminology. Pistols that are referred to as “autoloading” or “automatic” refer to the same thing. These are handguns that are capable of firing a round and loading the next one with a single trigger pull. It does not mean that constant pressure on the trigger results in a steady stream of fire, like you might see in a movie. I’m just going to refer to this type of firearm as a “pistol.” If you see me using this word below and in future lectures, this is what I mean. Let’s look at a schematic of a common pistol:

As you can see, the magazine is loaded into a space within the grip, which is a pretty common set up. The decocking lever (#10) is what is a safety mechanism. A firearm is cocked when it is ready to fire. When this is engaged, the hammer falls and a mechanism prevents the firing pin from striking a chambered round. Another safety mechanism would be a manual safety that is usually located on the slide (not pictured). This locks the slide so that the weapon can’t be fired. However, if the gun is cocked, it can be fired as soon as the manual safety is deactivated. Most modern pistols have one or the other or both. 

The main difference in these firearms is the ability to ready the gun for firing with each trigger pull. This is usually accomplished by a complex mechanism that harnesses recoil to bring the next round to the chamber. The spent portions of the bullet are ejected from the chamber when this happens. 

In contrast to revolvers, pistols tend to produce less exhaust and that can impact the appearance of wounding. As we mentioned in a previous talk, most newer and adequately maintained pistols are extremely resistant to accidental discharge. When such a situation is expected, one needs to refer that firearm to a ballistics laboratory to replicate it.

Rifles

In general, a rifle is firearm with a longer, rifled barrel. I’m not aware of any particular classification with regards to the barrel length, but there is a federal law that requires rifle barrels to be at least 16 inches. They are designed to be fired from the shoulder. Ammunition fired from rifles is usually larger than that of pistols, but not always. The biggest difference with respect to wounding from rifles is a higher muzzle velocity. We’ll touch more on that later. Rifle injuries are much less common than handgun injuries. 

Shotguns

The final type of firearm that you’ll see as a forensic pathologist in the United States is the shotgun. Shotguns are also shoulder-mounted firearms with longer barrels. Unlike the aforementioned examples, shotgun barrels have a smooth bore. This is because the ammunition they fire usually comes as pellets contained within a plastic cup. As you might imagine, wounding by shotguns is much different than wounding by the other types of firearms listed here.

Summary

With that, we’ve covered the major types of firearms that the forensic pathologist will encounter. Next, we’ll cover some basics of ammunition before we put the two together and see how they damage human tissue. Stay tuned!