Bullet Set Back

In his book, author, firearms instructor, and former Marine Paul G. Markel succinctly describes the journey through firearms proficiency from novice through “expert.” More importantly, he describes the attainment of "expert" not as indicative of the end of the journey, but a firm commitment to being a life-long student. Through my personal journey, I’ve had the opportunity to train, compete, and instruct in firearms disciplines… and there are still many things I learn every day. Although I’ve studied and written extensively on ballistics and considerations for personal and home defense ammunition, I recently "re-learned" a valuable lesson on bullet set-back that I’d like to briefly share with you.

For a number of years, I’d kept a 1911 in .45ACP as my home defense firearm and loaded it with 185 grain jacketed hollow-point cartridges. During this time, I’d followed the advice that I’ve often dispensed to “routinely train with the firearm, firing the personal defense ammunition to ensure 100% reliability and then thoroughly clean the firearm and the magazines."

Well, in the last two years, I switched my home defense firearm to a Sig P227 in .45ACP for the higher capacity (14 versus 8), the integral light rail, and the choice of Double Action or Single Action. Feeling confident with the 185 grain jacketed hollow-point cartridges, I kept the P227 loaded with this ammunition as well. At first, I routinely trained with this handgun/ammunition combination and found it to be as reliable as the 1911 that formerly held this role.

Another Step in the Journey

However, I "fell behind" on training with this system as often as I had in the past… opting to just unload the firearm and the magazines, clean and lubricate both, conduct a non-live-fire function check, re-load, and replace.

Recently, when I conducted this routine, I noticed how the Sig P227 failed to feed the first cartridge into the chamber which jammed the gun out of battery. Whereas a “tap” to the rear of the slide would normally return to the gun into battery, chamber the cartridge, and lock the slide and barrel to the frame, this cartridge was firmly jammed against the barrel’s feed ramp. For a home defense firearm that must be 100% reliable, I found this to be discomforting.

When I unloaded the gun, the cartridge in question fell free from the magazine and I noticed that it looked “a little different.” Well, it looked a lot different… and so did the second cartridge in the magazine. THEY WERE SHORTER!!!! This different dimension, the overall length, is one of the contributing factors that led to the jam. Other than the obvious concern that these cartridges could potentially jam my firearm when I needed it the most, I wondered how many other cartridges in the magazine were "shorter?"

With a digital caliper, I measured the overall length (from the end of the cartridge rim to the leading edge of the projectile) of the ammunition from the box which had not been chambered into the Sig P227 and they measured 1.206 inches. This measurement was consistent with all other un-chambered cartridges.

The measurement on the “short” cartridge was 1.103 inches! Although this is only about one-tenth of an inch, it was enough to jam the gun. So, I did a quick test with a “good” cartridge and chambered it repeatedly while taking a measurement between each iteration. I had found that each time I chambered the same cartridge, the bullet “set-back” about five thousands of an inch (0.005?). This could be caused by an insufficient crimp (pressure from the inside of the cartridge case against the outside of the projectile), the edge of the blunt-tipped jacketed hollow-point impacting the feed ramp, or both.

The bullet set-back was not only altering the external geometry of the cartridge as it fed from the magazine into the chamber which increased the potential for a jam, it was also changing the dimensions within the cartridge itself.

Internal Ballistics, Pressure, and Bullet Set-Back

If you recall from the Ballistics Series, there are four components to a metallic cartridge: the case, the primer, the propellant, and the projectile. Ideally, the composition/density of the propellant fills the otherwise empty volume of the cartridge case for a uniform ignition. Continue Reading »

Author: Howard Hall

Shotgun Ballistics

This 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 :

Primer – 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

 

Whereas 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.

Shot

 

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

Thank 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.

Attaining 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

 

Nomenclature: 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. 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: 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). (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: 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). (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. 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 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.