Tactical Skills for an Adventurous Life

Barrel Length, Trajectory, and Learning Your Zero

When I was younger, single, and didn’t have all that much responsibility in life, I was into cars and drag racing. There’ just something about the feeling of going fast that never got old, even if I did. When it came to engines, there was a saying: There’s No Replacement for Displacement.

It was almost always the guys driving around their big V-8s looking down upon my turbocharged four-banger. It was an indictment to engine technology. No matter what kind of whiz-bang tech is in the engine compartment, there really was no getting around that a larger engine produced more power.

Rifles work the same way. There’s simply no substitute for velocity.

This applies in a lot of different areas of ballistics and rifle technology, and I will get to them in due time. For now, I want to focus on the relationship between the length of a rifle’s barrel and velocity. More specifically, how that relationship affects your trajectory and marksmanship.

The Role of Barrel Length

There’s a lot of engineering voodoo that goes into making a rifle work well and fire accurately. You got a glimpse at some of that math in my discussion of barrel twist rates. But there really is a lot more out there. We’re going to focus on the physical forces that propel a projectile down the bore of the rifle.

First, the cartridge contains a fuel source sealed in a brass case. The projectile caps the unit at one end and the primer at the other. This way, it’s all self-contained. The reality is that you have all the components for a small rocket engine. When you lock the cartridge into the chamber,

The entire bore of the rifle becomes a rocket nozzle. As the fuel burns and releases high pressure gas, it expands in all directions. The side walls of the case are up agains the side of the much stronger chamber, so that’s not giving way unless the pressure of the expanding gas is higher than the strength of the chamber.

It does happen, especially when using high pressure pistol powder in a rifle by accident. Can you say ka-boom?

The bottom of the cartridge is up against the bolt face, which is mechanically locked into the rifle, which is braced against you. Combined, the weight of the rifle and you are heavy enough to provide resistance. So the only other way the gas can expand is down the bore and towards the muzzle.

Moving the Bullet

But there’s a catch. The bullet is in the way and sealed up tight against the rifling of the bore. Well, the pressure has to go somewhere. The bullet is the easiest thing to move at the moment, so the pressure continues to build until it overcomes the friction between the bullet and the bore.

The pressure continues to build to very high levels, over 51,000 PSI in the case of .223 cartridges. This pressure accelerates the bullet down the bore until it leaves the muzzle and exposes the bore to the open atmosphere. At that instant, the pressure disperses and the bore pressure drops back to equilibrium with the outside world.

Physics being what it is, the mass of the bullet leaving the gun along with the force of expanding gasses leaving the muzzle like a rocket nozzle is what you experience as recoil.

So as long as the bullet remains in the bore, it will continue to build speed. Therefore, more bore to travel through means more velocity for the bullet.

At least until a certain point.

Can You Have Too Much Barrel Length?

The short answer is yes, there can be too much of a “good thing.” Remember that there is a lot of friction between the bullet and the inner walls of the barrel. It’s a necessity in order to create the seal required for expanding gasses to push the bullet down the bore.

That’s all well and good as long as there’s enough pressure behind the bullet to overcome that friction. The problem is that we have a limited amount of fuel in the cartridge. If we had an infinitely long barrel, then there would be an equilibrium point. The bullet would move far enough down the bore that pressure would start to drop behind it. It might not return to ambient outside pressure, but it wouldn’t be high enough to keep the bullet moving.

This is effectively what happens with a squib load. There wasn’t enough pressure generated to push the bullet out of the bore.

All of that to say that you can increase the length of a barrel to a point and continue squeezing more velocity out of the shot, but at some point the bullet will start slowing down again.

Barrel Length Optimization

So back to my statement about engineering. The takeaway I want you to have is that there’s a lot of engineering that goes into finding the optimum amount of bore for a given combination of powder, caliber, and bullet weight.

This is all done as part of the initial engineering for a weapon system. It’s one of the reasons that a new small arm usually comes along with a new cartridge to go with it.

I brought up this chart in my article on choosing an AR-15 barrel. It’s based upon M855 fired from barrel lengths from 24 inches down to 5 inches. Notice that peak velocity occurs right around 20 inches of barrel length. That’s how the system was engineered from the start.

Velocity and Trajectory

One of the big misconceptions out there is that a longer barrel makes your rifle more accurate. This is incorrect. Barrel length doesn’t affect the mechanical velocity of your rifle.

At least not enough to worry about.

What’s actually happening is that the ballistic curve of the bullet becomes slightly flatter with more velocity. That means you need less sight adjustment or holdover in order to make a shot.

In other words, you perceive that it’s easier to hit a target when you have more velocity.

I have two ways to demonstrate this to you. The first is using the point blank zero concept.

Compare these ballistic calculator outputs, particularly the “Maximum PBR” range in the first column. The first is with a 20″ barrel firing M193, the second is a 16″ barrel, and the third is a 12.5″ barrel. These represent my three favorite barrel lengths for an AR-15. I estimated velocity at 3300, 3100, and 2900 respectively. I set the vital zone size to 8″ to represent the -0 zone of an IDPA target. Yeah, I know…IDPA is for pistol shooting, just go with me here because I like the nice round number.

So there’s one data point. Notice that the point-blank range for keeping the trajectory of the shot within +/- four inches of point of aim continues to steadily march inwards. But that’s not the whole story.

Barrel Length and Trajectory

If we’re being honest, the difference between a maximum point-blank range of 296 yards and 333 yards isn’t all that much. This is an ideal situation, though. If you notice the actual distances you have to zero your rifle at, annotated as Maximum PBR Zero, those aren’t common zeroing distances. I don’t know many ranges that let me zero my sights at exactly 232 yards.

So let’s look at this from the perspective of commonly used zero distances.

For this exercise, I’m going to use our same 20″, 16″ and 12.5″ barrel lengths firing 55gr M193. We’re also going to use a 25, 50, 100, 200, and 300-yard zero. I ran the numbers through JBM Ballistics and created these curves.

It’s not quite as clear, but you start to see the separation as the distances grow. For comparison, with a 100-yard zero, the shot fired from the 12.5″ barrel is dropping over a foot and a half more than the shot fired from the 20″ barrel at 500 yards.

Now, whether or not you would actually use a 12.5″ barrel at 500 yards is a very different question and outside of what we’re talking about here. To that point, remember that I didn’t consider the bullet’s energy in any of these charts or graphs. Punching a hole in a piece of paper is one thing, but actually having the impact cause significant damage is another.

This IDPA silhouette shows the point of impact for shots fired through a 20″ rifle zeroed at 300 yards

How it looks on a target

Looking at the charts and curves is one thing, but let’s really drive home the visualization. Let’s use the IDPA target again, with it’s nice even 8″ center score zone. In comparison, the A-Zone of a USPSA target is 11.03″, which is a little more awkward to work with.

I took each of plots from the zero distances and showed where the impact would be on the target. We’re assuming a center-mass point of aim for each shot, which I’ve highlighted in red.

Please keep in mind that these graphics are not perfect. I only did them for 55gr M193. Trajectories for different loads will change. I spent a lot of time getting things to scale, but this is ultimately more fo illustrative purposes than anything.

Notice on the 25-yard zero targets, both the 20″ and 16″ land hits at 400 yards (though just barely). The 12.5″ isn’t even close. Also notice the 300-yard impact point. As the velocity drops, even by a little, it steadily marches down the target.

What stood out to me with the 100-yard zero target is just how close in performance the 20″ and 16″ barrels were.

The 200-yard targets all show nice tight little clusters from zero to 200 yards right in center mass. This is likely why the 200-yard zero is so popular for practical shooters.

Notice that the cluster shrinks on each target as the velocity increases. This is what we mean by “flat shooting.” If you look closely, you’ll notice that the 12.5″ barrel starts sending shots higher at the 100 and 200-yard points. That’s because the rifle must be tilted at a steeper angle to actually achieve a 300-yard zero.

You’ll also notice that a 100-yard zero really isn’t all that useful as a point-blank zero since it’s always the highest impact point. That’s still useful, though, when you think about it.

A 100-yard zero means that you only ever have to think about holding your sights higher on the target. If you look at the 300-yard zero, you can easily imagine a scenario where aiming for a head shot means sending rounds over the top and missing completely. With a 100-yard zero, you know that your shots will only ever fall lower into center mass.

The 20″ barrel keeps the tightest cluster across the board. You can also see several instances where the 20″ barrel keeps a 400 yard impact on the target, whereas the 12.5″ barrel is totally missing.

Wrapping Up

I hope you enjoyed this little exercise. There are three big takeaways you should keep from this:

  • The 50/200 or 25/300 zero is a myth. The charts all clearly show that there are different impact points. Sure, they are pretty close, especially with a 20″ barrel, but do not think that because you zeroed at 50 yards it also means you’re dead on for 200. Always verify at the further distance if that’s your goal.
  • Overall, the 200-yard zero appears to be a good all-around zero for AR-15s firing the 55gr M193 cartridge. The 50-yard zero is close as well. You can definitely see why it’s so common.
  • The 300-yard zero actually looks like a decent solution for combat, assuming you can always aim center mass. Aiming center mass means that you’re likely to land an incapacitating shot from 0 to 400 yards. You can see why it grew in popularity as the default battlesight zero when everyone was running around with fixed iron sights.

This should also drive home the importance of knowing your zero. Picking a solid point-blank zero to minimize the amount of holding over you need to do is important, but you also need to be aware of where your impacts will fall.

With all of that said, you can see why I consider the 16″ barrel to be a good all-around length, but the 20″ is still king for flat shooting.

Thanks for reading, and please don’t hesitate to ask any questions down in the comments!

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Excellent post! As an engineer who goes so far as to chrono his pellet guns (!), I really dig the graphs and graphics.

One quibble, though. On the 300-yard chart, if you count the divisions, you’re at 11 when you hit 10″, so the divisions are at 0.91″.

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