Internal Ballistics Part II – Mechanical Precision

Welcome 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

Anyone 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.

Freebore: 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.

Barrel 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.

Barrel 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.

We’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 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.