Shotgun Ballistics

Shotgun BallisticsThis article answers a reader’s question and discusses shotgun ballistics.

“What about Shotguns and Shotgun Ballistics? The entire Ballistics Series was great and it was the first time that the science behind shooting was explained in a way that made sense. I’m considering a shotgun for home defense, so could you cover Shotgun Ballistics in one of your upcoming articles? Thank you.” – Brian in Temecula

Brian, thank you for the e-mail and for the opportunity to give the venerable shotgun the attention it deserves. Although there are many similarities between shotgun ballistics and rifle/pistol ballistics, the differences are significant enough to focus solely on the shotgun. What makes shotguns unique among other firearms is the wide variety of projectiles that can be fired from the same platform. This includes everything from a slug and a sabot round through buck shot, bird shot, and a number of less-than-lethal options. Since slugs and sabot projectiles are single solid projectiles, their ballistic characteristics are very similar to the pistol and rifle projectiles covered in the Ballistics Series. Therefore, I will focus the majority of this article on buckshot and birdshot.

Internal Ballistics

If you recall, our discussion on internal ballistics focused on the characteristics of the firearm, the cartridge, and the initial actions in the firing sequence that occur within the confines of the firearm. For this discussion, we’ll start with the cartridges, known as shot shells, and their components :

shotgun shell case cutawayPrimer – similar to rifles and pistols, the shot shell primer is a small metal cup that contains an explosive mixture that, when struck by the firing pin, sends a small flame through the base to ignite the propellant.

Base – brass, steel, or aluminum, the base is a multi-function element that houses the primer, binds the hull, and provides rigidity to interact with the extractor and ejector to ensure firearm function.

Hull – polymer, plastic, or paper, the hull ensures the proper functioning of the shot shell by holding the individual components together behind a crimp. Once the propellant is ignited, the hull expands to the diameter of the chamber and ensures a sufficient gas seal to send the wad and shot forward.

Propellant – similar to rifles and pistols, the propellant, once ignited, produces the gas expansion required to fire the projectile(s).

Wad – Different from rifles and pistols, a shot shell requires a wad in order to: (1) provide a small internal compartment to contain the propellant and keep it separate from the shot or projectile; and (2) provide a buffer that absorbs shock and minimizes deformation of the shot as it accelerates from rest to initial velocity.

Shot – From a collection of very small to large lead or steel pellets through slugs, sabots, or less-than-lethal projectiles, the shot is a single or group of projectiles delivered from the firearm to the target.

Gauge and Chamber

shotgun-gaugeWhereas rifles and pistols are designated by the caliber of the projectiles they fire, shotguns are designated by the shotshell gauge. While most shooters have heard the common gauges of 12, 20, 28, and .410, few know the origin of the term. A shotgun’s gauge is determined by the number of round projectiles of equal diameter that can be subdivided from one pound of lead. For example, a 12-gauge barrel was designed around the fact that one pound of lead could be divided into twelve 0.727-inch lead balls. The same methodology is true from the 4 gauge down to the 28 gauge. .410 is the exception to the rule since it was simply a determination of the diameter of the bore in fractions of an inch.

While the gauge determines the inside diameter of the bore, the chamber designation determines the maximum length of the cartridge. Most shotguns can chamber a cartridge of either 2-3/4 inches or 3 inches while others can handle a 3-1.2 inch shell. Together, the gauge and chamber determine the maximum volume of shot or size of the projectile.


Whether lead, steel, or another material, the shot is a volume of small diameter round projectiles. In the preceding section, we discussed how the gauge and chamber determine the maximum volume of shot a cartridge can contain, the diameter of the shot itself will determine how many projectiles can fill that volume. Ranging from #12 shot (at 0.05 inches per ball) to OOO, or triple-ought (at .36 inches per ball), each have their own application. Skeet shooters engaging fast-crossing targets at close range desire a large quantity of small pellets, so they typically choose a #9 shot. Conversely, trapshooters engage targets moving quickly away from the shooting position, so they choose either a #8 or a #7.5 shot with the requisite momentum to catch-up to and break the clay pigeon. The smaller shot is ideal for breaking clay pigeons or taking-down game birds without destroying their feathers or meat, but the small shot retains insufficient energy to take-down a larger game animal or subdue a felon. In these cases, the larger buckshot, slug, or sabot is chosen.

Internal/External Ballistics – Smooth-Bore, Forcing Cone, and Chokes

Break-open, pump, and semi-automatic shotguns share the same cycle of operations and functions as the rifles and pistols I covered in Internal Ballistics – Part I. Very briefly, as the firing pin strikes the primer, a small flame is sent through the base and into the hull, which ignites the propellant. The resulting rapid gas expansion pushes the plastic hull against the inside of the chamber causing a seal from which the wad and its contents can only push forward down the bore.

Shotguns, however, differ from pistols in rifles in three main areas: smooth-bore, forcing cone, and chokes. While some shotguns are manufactured with rifled bores, many more are manufactured with smooth bores. The rifled bore is designed primarily for the rifled slug and sabot rounds and produces the gyroscopic stability required to send the projectile to the intended target. Technically, birdshot and buckshot can be fired through a rifled bore, but the rifling will “spin” the plastic wad which will translate the centrifugal force to the shot and open the shot column into a “V,” leaving a large gap in the center of the shot spread. Therefore, birdshot and buckshot are meant to be fired through smooth-bores.

Since the chamber is larger than the bore, something needs to gradually “step-down” from the hull diameter to the inside diameter of the bore. This is the forcing cone. The length and shape of the forcing cone will affect the efficiency of the wad and shot traversing the length of the barrel. Too-short of a forcing cone can exert undue stress on the lead shot and cause deformation while too-long of a forcing cone may not generate enough pressure for sufficient initial velocity.

Once the wad and the shot exit the barrel, the volume of shot expands in both length and width. The size of the shot, shape of the forcing cone, and constriction provided by the choke work together and determine how much the shot column spreads as it travels down-range. As this shot column passes through a two-dimensional plane at any distance, it leaves a pattern. The following graphic demonstrates the spread of the shot pattern at different distances…. Read more >>

Posted by Howard Hall


Internal Ballistics Part III – Ammunition Characteristics and Safety

Internal Ballistics Part III – Ammunition Characteristics and SafetyThank you for your continued interest in the study of ballistics. So far, we’ve covered the broad framework of firearm ballistics followed by two in-depth discussions regarding internal ballistics (firearm function and mechanical precision). In this article, I will conclude our discussion on internal ballistics by focusing on ammunition characteristics and safety.

In contrast to firearm function, which focused on the mechanical interaction of individual firearm components, understanding ammunition requires a focus on the thermodynamic conversion of chemical potential energy into kinetic energy. At the center of this energy conversion is the important concept of pressure. Rapidly expanding gasses create the pressure required to expand the cartridge case against the inside of the chamber, propel the projectile forward, and cycle semi-automatic weapons.

blowupAttaining consistent and safe pressure is the key to precision and reliable firearm function. Too little pressure causes firearm malfunctions and reduces both precision and terminal ballistic performance (bullet expansion, etc). Too much pressure can cause injury to the shooter and damage the firearm.

In addition to precision and function, three very important ammunition concepts relate directly to safety: the misfire, the hang-fire, and the squib-load. A misfire occurs when the firing pin properly strikes the primer, but the propellant does not ignite. A hang-fire occurs when there is a perceptible delay between the firing pin striking the primer and propellant ignition. A squib-load occurs when the firing pin strikes the primer and the propellant ignites but produces insufficient pressure to expel the projectile from the bore. These will be explained in greater detail below.

In order to fully understand how energy conversion, gas expansion, and pressure interact to fire a projectile, let’s take a detailed look at ammunition components.

Ammunition Components

Diag Bullet CutoutNomenclature: while often referred to by many different names (ammo, bullets, shells, etc.), ammunition is technically an enclosed system of components (case, primer, propellant, and projectile). For clarity, I will refer to the system of components as a cartridge.

Case: In general terms, the case holds all of the components together. Rifle and pistol cases can be made from brass (gold color), aluminum (gray), or steel/nickel (silver). Four main parts of a case are the primer pocket, rim, wall, and mouth. The open volume inside of the case determines the maximum amount of propellant the cartridge can hold. G 21 Case 2 Function – as the propellant ignites and produces rapid gas expansion, pressure forces the case walls and mouth outward to form a tight seal against the inside of the chamber. Forming this tight seal is important for two reasons: (1) it directs remaining pressure forward to efficiently propel the projectile; and (2) it facilitates the rearward momentum required to cycle most semi-automatic firearms. Loss of pressure due to cracked or malformed cases can cause firearm malfunction or injury. What You Should Know – All shooters should routinely check a portion of their spent cases for signs of overpressure (cracks, bulges, elongation, etc.). Spent shell cases that show signs of overpressure indicate a possible firearm malfunction.

Primer - Firearm MalfunctionPrimer: The modern percussion primer consists of a small metal cup that contains a pellet of sensitive explosive material secured by a paper disk and a brass anvil. A strike from the firing pin on the center of the cap compresses the primer composition between the cap and the anvil. This causes the composition to ignite. Holes or vents in the anvil or closure cup allow the flame to pass through the flash hole in the cartridge case and ignite the propellant. Primers are constructed to be rugged enough to prevent unintentional detonation while retaining malleability to assure consistent ignition. Function – the primer plays a major role in the ignition sequence. Conversely, it can either be the source of or the key to diagnosing firearm malfunction. What You Should Know – (1) when firearms and cartridges are functioning properly, routine inspection of spent cases should indicate uniform firing pin indentations in the center of the primer. The same routine inspections can also identify excessive pressure (primer flow – the primer is literally either pushed out of the primer pocket or is spread along the base of the cartridge). Primer 2(2) in the event of a misfire (no primer ignition), inspect the primer to determine if the firing pin either struck the primer off-center (improper headspace or extractor tension), did not strike the primer with enough force, i.e. a “light strike” (could indicate a dirty firing pin channel, broken firing pin, or improper headspace), or the cartridge malfunctioned (primer set too deep in the pocket or the primer itself is bad). In the latter case, if the problem persists among multiple cartridges, attempt to fire a different brand of ammunition through the same firearm to determine if it is an ammunition malfunction or firearm malfunction. If it is the ammunition, sequester the remainder of the box, note the lot-number, and contact the manufacturer. (3) hang-fires and squib loads are not normally attributable to primers.

Propellant - GunpowderPropellant: more commonly known as “gunpowder,” propellant has come a long way since its first use in firearms in the early 1200s. Most modern propellants are double-based nitrocellulose compounds produced in a variety of physical dimensions (densities and shapes). These densities and shapes determine how quickly the propellant burns. Function: Three components are need for combustion: kindling temperature, combustible material, and oxygen. In conjunction with the primer’s flame, propellant contains all three. Through thermodynamic conversion, the mass of the propellant is changed into heat energy. During deflagration (rapid burn), heat energy is converted into massive gas expansion which creates the required pressure to complete the sequence. What You Should Know – (1) in the event of a hang-fire, keep the weapon pointed in a safe direction for at least 30 seconds before opening the action to remove the cartridge. It is difficult to determine exactly what causes a hang-fire, but the most likely causes are propellants that have degraded due to age or exposure to moisture (water, gun oil, etc). Propellant 2 (2) in the event of a squib-load, you will most likely hear an audible “pop” but not feel the full recoil of weapon. In most cases, the cartridge was loaded with either insufficient or no propellant at all. Initial pressure may expel the projectile from the cartridge case but leave it lodged in the barrel. Therefore, if you detect a squib-load while firing… stop firing immediately! With a projectile lodged in the barrel, firing the next round would create a dangerous overpressure in the firearm that could cause serious injury and damage major components of the firearm.

Projectile: Projectile construction, shape, and weight are most applicable to discussions on external and terminal ballistics, which I’ll cover in the next two articles. For internal ballistics, however, projectile shape and weight contribute to both mechanical precision and “impulse” or felt recoil. Function: Robert Rinker, in his book “Understanding Firearm Ballistics,” lists the following conclusions in regard to projectile characteristics and mechanical precision: (1) the longer the projectile in proportion to its diameter (caliber) the more barrel twist is needed to stabilize it; (2) A very long nose projectile is good for drag reduction and aerodynamic efficiency, but is harder to stabilize; (3) A longer and heavier projectile can be stabilized at the same rifle twist by increasing velocity; (4) A slower projectile requires a faster rifle twist to stabilize it. In regard to impulse or recoil, it is a basic physics principle that it takes a greater force to move a heavier projectile from zero velocity to initial velocity than it does a lighter projectile. Therefore, firing a heavier projectile will result in a greater impulse/recoil. Setback 9mm What You Should Know: In most metallic cartridges, the projectile is held in place within the mouth of the case simply by friction. This friction is created by applying a light crimp on the outer edge of the case mouth. In some instances, the act of feeding (stripping the cartridge from the magazine) and chambering (pushing the cartridge into the chamber) can cause “projectile set-back. Although rare, this is caused by the projectile tip contacting the feed ramp on its way into the chamber. For shooters who load and fire cartridges in the same action, this is less of a concern. For personal defense cartridges, however, the same cartridges can be chambered and unloaded numerous times before being fired, and thus the same projectile contacts the feed ramp on numerous occasions. In the event the projectile is literally “pushed into” the cartridge case, the volume within the case is reduced and the potential for overpressure is increased. In rare circumstances, this overpressure can cause injury to the shooter or damage to the firearm. If you chamber and unload your personal defense ammunition multiple times, please conduct a visual inspection to ensure that you are not “setting-back” the projectiles.

Putting it All Together

Undoubtedly, your intended use (target practice, competition, hunting, self-defense) will ultimately determine which cartridge you purchase. However, understanding cartridge characteristics and functions as they relate to internal ballistics and pressure can help you become a better and safer shooter.

Next month, we will cover external ballistics and the factors that affect a projectile’s flight to your intended target. Until then, stay safe and shoot straight!

First Published at Aegis Academy

About Author

– Howard Hall

Range Master

Howard HallHoward has served for nearly 20 years in the Marine Corps. He has served as a Platoon Commander, Company Commander, Battalion Executive Officer, Regimental Operations Officer, and Battalion Commander. He has multiple combat tours to include serving as a military transition team member in Fallujah. He is an NRA Certified handgun instructor and holds numerous Marine Corps training credentials. An active competitor in action pistol (United States Practical Shooting Association), long range rifle (NRA F-Class), and shotgun (Amateur Trapshooting Association, National Skeet Shooting Association), howard has earned numerous accolades and medaled during DoD competitions with the 1911 platform in bulls-eye shooting.

Internal Ballistics Part II – Mechanical Precision

Internal Ballistics – Part II – Mechanical Precision

internal ballistics, Howard Hall, Aegis Academy, Gun Training, Firearms TrainingWelcome back! Thank you for your interest in ballistics. So far, we’ve covered a broad overview of the different types of ballistics, weapon function, and the cycle of operations. (Ballistics – Making Every Shot Count; Internal Ballistics Part I – Cycle of Operation and Firearm Function) This month, I will continue to cover internal ballistics. Defined as the combination of actions and reactions within a firearm as they affect a projectile’s movement to the end of the barrel and ultimately affect a bullet’s flight to target. With volumes of information available, I’ve chosen to focus this month’s column only on the mechanical aspects of the firearm and will cover ammunition details next month.

When I first started shooting bulls-eye competitions, I was next to a shooter using a 1911 with a six-inch barrel. In my naiveté, I asked him what was the benefit of his 6” 1911 compared to my standard 5” 1911? He humorously replied… “I’m an inch closer to the target than you are!” There is a lot more to accuracy and precision than barrel length alone.

Precision vs. Accuracy

accuracy precision, internal ballistics, Howard Hall, Aegis Academy, Gun Training, Firearms TrainingAnyone who has shot a wide variety of firearms has probably noticed some just seem “more accurate” than others. While the concept is sound, the nomenclature is a bit misleading. To be clear, it requires both accuracy and precision to consistently place shots within the desired area of a target. The term “precision” refers to the mechanical qualities of a firearm that combine to consistently place projectiles in a small group on target. “Accuracy,” on the other hand, refers to the shooter’s ability to harness the firearm’s intrinsic precision and place a tight group of shots in the desired portion of the target. The shooter alone controls accuracy through refined technique, stance, gun fit, grip, sight alignment, sight picture and follow-through. This article will focus on mechanical precision.

Firearm precision is simply a matter of mechanical repeatability. By this, I mean the mechanical components of the firearm must interact in a controlled and consistent manner to launch the projectile exactly the same way. If these mechanical components do not interact in a consistent manner, the net result may negate all other factors, thus sending projectiles outside of the desired area within the target. For example, a shooter may have selected the correct ammunition matched to the weapon and exercised a highly refined shooting technique, only to find the point of impact on target is inconsistent. No combination of applied shooting principles can overcome a lack of mechanical precision.

Firearm Construction

Tolerance is a key contributor to firearm precision. It is defined as the dimensional relationship between moving mechanical parts, or how well the different firearm components fit together. While it is obvious the moving parts within a firearm require enough clearance to perform their functions, excessive gaps can cause inconsistent alignment and reduce precision. Mass produced firearms tend to cost less due to the manufacturer’s ability to fabricate thousands of interchangeable parts in a single run to construct their firearms. In general, these firearms tend to be inherently “more” functional but “less” precise due to the lower tolerance engineered into each component. Note that the terms “more” and “less” are generalizations and vary among manufacturers. Conversely, firearms built with high tolerances and hand-fit components seem “tighter” and result in a lower manufacturing volume, higher cost and greater maintenance requirements to ensure reliable function.

Anyone who has had the opportunity to shoot an old 1911 may have noticed they “rattle” when you shake them… yet they are still VERY accurate. The rattling sound is typically the physical contact between the slide and the frame where there is a low tolerance due to high volume production and the need for consistent function. However, this unique pistol remains accurate due to a higher tolerance where it is most needed. The area where the rear of the barrel locks into the slide and the front of the barrel is secured by the barrel bushing.

Each shooter must decide their own threshold of precision, cost, and reliable function in their firearm selection. Below, I will describe a few other key characteristics of firearm construction as they relate to both precision and function.

Headspace: As the cartridge is stripped from the magazine or manually inserted and the firearm chambers and locks, headspace is defined as any measurable gap between the cartridge base and the bolt face, breech or receiver. Excessive headspace allows the cartridge to “move” in relation to the bolt, breech face or receiver and produce inconsistent ignition or allow expanding gasses to escape. This influences the travel of the projectile into the bore. Extractor quality, shape, construction, and tension play a key role in reducing headspace. Heavily used guns can also experience erosion on the face of the bolt, breech or receiver. In any case, improper headspace degrades both precision and function.

internal ballistics, Howard Hall, Aegis Academy, Gun Training, Firearms TrainingFreebore: This is the distance between the foremost portion of the exterior diameter of the projectile and the beginning of the rifling. Upon initial gas expansion, this is the distance a projectile must travel before engaging the rifling. Testing has shown that precision is increased when a chambered and locked cartridge presses the projectile against the rifling. However, this is not feasible for most semi-automatic weapons due to the maximum cartridge length afforded by the internal dimension of magazine. Precision rifle shooters who load their own ammunition for maximum precision tend to gauge the freebore and maximize the overall length of the cartridge to set the projectile against the rifling. For this discussion, we need to realize that “shorter” cartridges with excessive freebore can shear portions of a bullet jacket as it slams into the rifling, which will significantly affect its external ballistics.

Barrel length: Each shooter must select a firearm with a barrel length that suits their individual needs. Shorter barrels benefit those who desire lighter and more concealable firearms. Longer barrels tend to benefit shooters who don’t mind the added weight and are interested in greater precision and accuracy. Longer barrels inherently present a longer sight radius, which improves accuracy. Concurrently, increased precision is also produced by the greater duration from which the barrel influences the travel of the projectile before it is released into the atmosphere. Longer barrels also provide a greater initial velocity due to the duration of the powder burn, gas expansion, and pressure build-up.

internal ballistics, Howard Hall, Aegis Academy, Gun Training, Firearms TrainingBarrel Twist: Rifling in a firearm barrel is a series of helical grooves that rotate a projectile along its longitudinal axis as it travels along the barrel. This produces the gyroscopic stability required for the projectile to consistently travel to the target. Twist can be measured by pulling a cleaning rod with a cloth patch through the barrel from the chamber to the crown. Allowing the cleaning rod to spin freely, the number of complete rotations it makes in one inch is the ratio of twist. If the cleaning rod makes one complete turn in 7 inches, it is considered a 1:7 twist. Please see Chris White’s excellent article on matching ammunition to barrels for more explanation: Barrel Twist and Bullet Weight

Barrel Effects: While shooting a firearm, the most noticeable effects are impulse (the bang) and the subsequent recoil in conjunction with the movement of the slide or bolt. However, few shooters realize there is a tremendous amount of activity within the barrel itself. The rapid gas expansion that creates pressure to propel a projectile down the bore causes the barrel to expand. Concurrently, the rifling that forces the projectile to spin as it proceeds down the bore also creates a torque in the opposite direction, causing the barrel to “twist” in the opposite direction of the rifling. Meanwhile, the impulse and rapid gas expansion that sends the projectile forward also causes the barrel to vibrate and literally whip. Picture yourself holding the end of a taught rope, quickly moving the end in your hand, and watching the wave move down the rope. With all these forces acting on the barrel as the projectile moves forward, mechanical precision can only be attained if the projectile exits the moving barrel at the exact same time in its movement. The amount of barrel movement is affected by the quality of the barrel construction material, barrel thickness and the points of contact with the barrel (in rifles, this is considered bedding or free floating to ensure there is limited contact with the stock). In general, lighter and thinner barrels are more affected by expansion, twist, vibration and whip. Conversely, heavier and thicker barrels are less affected and enhance precision.

barrelcrowns, internal ballistics, Howard Hall, Aegis Academy, Gun Training, Firearms TrainingBarrel Crown: While there are many forces exerted on the projectile as it travels down the barrel, the very last influence occurs as the projectile makes its very last contact with the barrel. The shape of the barrel crown will determine this last influence as the projectile enters the atmosphere and residual gas expansion makes its last push against the base of the projectile. If the crown is machined in an uneven manner or if there is some damage to the crown, the last “gas push” will be unevenly distributed on the projectile causing unwanted yaw that will send it away from the intended area of the target. Often, the barrel crown is recessed with ample material on the outer part of the barrel protecting the inner bore where the crown resides. Even if the barrel crown is not damaged by misuse or impact, heavy use combined with a lack of cleaning can erode the surface of the crown and cause an uneven gas expansion that leads to a lack of precision.

internal ballistics, Howard Hall, Aegis Academy, Gun Training, Firearms TrainingWe’ve only scratched the surface on this topic. I’ve only selected some topics I believe to be the most compelling and interesting to our shooters. If you are interested in gaining a deeper understanding of internal ballistics, I very highly recommend Robert A. Rinker’s book “Understanding Firearm Ballistics.” He provides a straight-forward no-nonsense approach that appeals to shooters looking for easy to understand concepts as well as advanced mathematics and physics principles. Find more information on Rinker’s book here.

Stay safe and shoot straight!

First Published at Aegis Academy

About Author

– Howard Hall

Range Master

Howard HallHoward has served for nearly 20 years in the Marine Corps. He has served as a Platoon Commander, Company Commander, Battalion Executive Officer, Regimental Operations Officer, and Battalion Commander. He has multiple combat tours to include serving as a military transition team member in Fallujah. He is an NRA Certified handgun instructor and holds numerous Marine Corps training credentials. An active competitor in action pistol (United States Practical Shooting Association), long range rifle (NRA F-Class), and shotgun (Amateur Trapshooting Association, National Skeet Shooting Association), howard has earned numerous accolades and medaled during DoD competitions with the 1911 platform in bulls-eye shooting.