Ballistic Basics – Velocity and Barrel length

Ballistics is a broad topic and the best way to understand it is to break it down and consume it in pieces, sort of like eating an elephant, no matter how you slice it, it comes down to one fork-full at a time.

I decided to start with defining the association between barrel length and velocity because it is a basic principal, an easy to plot linear scale and a good foundation to understanding how the other forces interact to provide the energy that moves a bullet from the chamber to the target. Plus, it is the one that most effects the energies delivered to the target.

We will use the 7.62 mm FMJ, steel jacketed bullets (U.S. Military designation M80) with a specified mass of 9.6 g (147 gr) and a velocity of 847 m/s ± 9.1 m/s (2780 ft/s ± 30 ft/s) as an example since this is the standard the NIJ uses to test rifle caliber performance for Level III and Level IV certification. This is out of a weapon with a 16′ barrel, which is the common barrel length for most factory-build military style .308 rifles, as well as for most .556 AR weapons. The standard 16″ barrel is actually a legal requirement on manufactures and the gas systems of these weapons is designed to perform best at that length.

If you are reading this, then you know that a factory-built military-style rifle is about as common as a stock Harley Davidson. Rare, few leave the dealership for the first time without a bag of skull somethings in tow to be attached to the bike. With rifles we tend to test them as bought and then improve upon them as we better understand their capabilities. Changing the barrel length to get more bang for the buck is one of the easiest things to do. With modern rifles and ammunition the accepted minimum barrel length for the complete consumption of the powder charge is about 9″. Anything less than that is going to produce some significant flame out of the business end of the barrel. Two downsides to this – for hunting you are wasting significant energy which translates into a slower bullet and less terminal velocity at point of impact and two, and the more important, in combat you are doing the same while adding the bonus effect that you are going to look to the enemy like you are firing a cannon (the Dopler Effect) and you will most certainly mark yourself as their first target to eliminate. While most game will only realize and care about this latter part if you miss, you can be assured that any of your companions will take delight in using it to make you infamous on social media. After all, what are friends for?

With newer and faster rounds coming to market and gaining widespread support even the NIJ is looking to change their certification standards and one of the changes they are considering is making the standard barrel length for testing 18″ or 20″. BTW, the military versions of these rifles were designated to be 20″ barrels to improve shooting accuracy, but testing has shown shorter barrels to just as accurate in the hands of an accomplished marksman.

In any case the longer barrel keeps the gases trapped behind the bullet longer, so that when it leaves the muzzle, it comes with somewhat higher velocity — usually somewhere around 3,200 feet per second, depending on bullet weight, powder charge, barrel wear, etc. It’s estimated that each inch of barrel lost will cost 25-50 fps in velocity, so you can expect around 3,000-3,100 fps from a 16″ barrel, and about 2,900-3,000 from a 14.5″ (tests with the 5.56 x 45 M193 ammo chronographerd at about 78 deg F; actual velocities vary with load, atmospheric pressure, temperature, barrel condition, and the shooters positioning). Velocity measurements in NIJ certification are taken at 8.25 feet from the target (to keep the spall and frag from destroying the measuring equipment.)

So, to a reasonable point, the longer the barrel, the more efficient and complete the powder burn, the more energy transferred to the bullet. The result is it travels faster and delivers more kinetic energy at its terminal velocity – when it hits the target. Rounds that are defeated by Level III body armor from a 16′ barrel may pierce the armor when the barrel length is increased to 20″ and all other conditions remain constant. It is obvious that velocity is directly related to terminal energy (striking the target), just look at the damage cause by vehicle impacts at school-zone speeds as opposed to freeway speed. Momentum – speed plus weight. Be safe out there.