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 coming from the guys driving around their big V8s looking down upon my turbocharged four-banger. No matter what kind of whiz-bang tech is under the hood, there really was no getting around the inherent advantage of larger engines.
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 a rifle’s barrel length and velocity.
More specifically, how that relationship affects your trajectory and marksmanship.
The Role of Barrel Length
There’s a lot of engineering voodoo going into accurate and reliable rifles.
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.
For this article, we’re focusing on the physical forces propelling a projectile down the bore of the rifle.
Firstly, understand that the cartridge contains a fuel source sealed in a brass case.
The projectile sits atop the fuel source and seals one end of the case. At the other end, you’ll find the sealed bottom of the case and a small hole for the primer.
In this configuration, each cartridge is self-contained. When you think about it, you have all of the components for a small rocket engine. By locking 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 press against the inside of the chamber, which is made of a much stronger metal. Since the walls of the chamber aren’t going to “give way” to expanding gas. Well, that’s not to say it never happens, but it’s considered a catastrophic malfunction when it does.
Such failures usually happen when someone mistakenly loads high-pressure pistol powder into a rifle by accident.
The bottom of the cartridge presses against the bolt face, which is mechanically locked into the rifle. Any energy exerted here transfers to the rifle and through the stock, where it ultimately comes up against you.
When the combination of chamber walls, bolt face locked to the receiver, and the shooter are taken into account, the only direction that expanding gasses can expand is down the bore and towards the muzzle.
Moving the Bullet
This is where things get interesting.
The bullet itself sits between the expanding gasses of burning powder and the muzzle. All of that expanding pressure has to go somewhere, and the bullet is the easiest thing to move at the moment.
The pressure continues to build until it overcomes the friction between the bullet and the bore. Again, the capacity to hold pressure isn’t infinite here. The chamber and bolt face only need to resist more pressure than the bullet wedged into the bore.
We’re talking significant amounts of pressure in the realm of 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 gas 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 combined 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 continues to build speed. Therefore, when there is more bore to travel through there is more potential to gain velocity.
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. The seal between the bullet and bore is necessary, or else the expanding gasses behind the bullet would blow past it.
All of that friction is fine as long as there’s enough pressure behind the bullet to overcome it.
The problem comes when from the limited amount of fuel in the cartridge. If we had an infinitely long barrel then there would be an equilibrium point. By that, I mean that 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, either.
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 unless you increase the available fuel to burn with a larger cartridge.
Optimizing Barrel Lengths
Let’s come back to my original statement about engineering voodoo.
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.
All of these calculations happen during a weapon system’s initial development. It’s one of the reasons that a new small arm usually comes along with a new cartridge to go with it.
When you look back over the history of the AR-15, you’ll find that we didn’t just end up with a 55gr .22 bullet fired through a 20″ barrel.
The design stemmed from decades of research and experimentation, with the final specification being a 55gr bullet launched at 3300 feet per second and penetrating a steel helmet at 500 yards.
That requirement balanced against restrictions on how much the rifle could weigh.
The end result is engineering voodoo to deliver the required performance to specific ranges, and no more.
Velocity and Trajectory
One big misconception 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 flatter as velocity increases. In other words, 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 at 3300 FPS, the second is a 16″ barrel at 3100 FPS, and the third is a 12.5″ barrel at 2900 FPS. These represent my three favorite barrel lengths for an AR-15.
I used 8″ as the “vital zone” for point blank range 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.
Notice that the point-blank zero distance for keeping the trajectory of the shot within +/- four inches of point of aim continues to steadily march inwards as the barrel length decreases.
That’s one data point, but it’s not the whole story.
Charting 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 idealized situation, though.
If you notice the actual suggested zero distances for these rifles, 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 using the same 20″, 16″ and 12.5″ barrel lengths firing 55gr M193. I’m also charting a 25, 50, 100, 200, and 300-yard zero for each one.
I ran the numbers through JBM Ballistics and created these curves.
It’s not quite as clear since these charts only go to 500 yards, but the separation between each curve gets more distinct as the distances grows.
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.
How it looks on a target
Looking at charts and curves is one thing, but let’s really drive home the visualization.
I took each of plots from the zero distances and showed where the impact would be on an IDPA target. We’re assuming a center-mass point of aim for each shot, which I’ve highlighted in red
These illustrations are not perfectly to scale, though they are pretty darn close.
I only did them for 55gr M193 from our three barrel lengths.
Trajectories for different combinations of bullet and barrel change significantly. But you’ll get the point.
On this 25-yard zero illustration, 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.
Next, let’s look at the 50 and 100-yard targets. What stood out to me with the 100-yard zero target is just how close in performance the 20″ and 16″ barrels were.
Now look at the 200-yard targets. They all show nice tight little clusters from zero to 200 yards right in center mass. This is why the 200-yard zero is so popular for practical and military shooters.
In all, this 200-yard zero is a really good all-around point for the 5.56 cartridge used at practical distances out to 300 yards.
Lastly, look at the 300-yard targets. 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 ballistic arc for a lower velocity must curve higher to zero at distance. In other words, to get a zero at 300 with a short barreled rifle, you have to tilt the whole thing upward.
The 100-yard “All Purpose Zero”
Compared to the targets from 25, 50, 200, and 300 yards, you probably noticed that a 100-yard zero really isn’t all that useful as a point-blank zero.
On all of the targets, the 100-yard reference 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 scenarios 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’ll also see several instances where the 20″ barrel keeps a 400 yard impact on the target, whereas the 12.5″ barrel is totally missing.
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, while not a perfect match, is fairly close.
- The 300-yard zero 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!