AR-15 Rifle Barrel Length – Does It Even Matter? Maybe NOT

Tom Mchale tries to answer that age-old question : If you buy the wrong barrel length will you fail to ever hit another target?

These Aero Precision barrels are identical, except for Barrel Length. Same steel, same manufacturing, same rifling.
These Aero Precision barrels are identical, except for Barrel Length. Same steel, same manufacturing, same rifling.
Tom McHale headshot low-res square
Tom McHale

USA –-( Of all the things you agonize over when choosing an AR-type rifle, should barrel length be on the list?

How much does it matter whether you choose a 16, 18, or even 20-inch barrel? If you buy the wrong one, will you fail to ever hit another target? Will the bullet even make it out of the fiery end before falling to the ground?

Will General ‘Mad Dog’ Mattis ever compete on Dancing With The Stars? Here, we’ll explore the answer to these, and more, important questions.

AR-15 Rifle Barrel Length

Just to set the stage, we’re talking about service rifles here, not fancy bench rest stuff where competitors have to call their shots for either the right or left eye of a mosquito or other ectoparasite at distances of 800 yards. So, in the context of recreation and defensive use, let’s take a look at the impacts (or possibly lack thereof) of choosing from the most common length barrels. With just one “just for fun” exception, we’ll stick to non SBR barrels that are available without the need for sending Uncle Spendy extra cash for tax stamps: 16″ barrels, 18 barrels, and 20 barrel inches.

Oh, you might notice a fourth one in the photo. That’s an Aero eight-inch300 AAC Blackout barrel. We’ll try that in comparison to a 16-inch model too.

Before we get into testing, some background info is in order.

There are two big factors that impact velocity of a bullet in Barrel Length:

First, in a perfect, frictionless environment, as long as the pressure behind the projectile is increasing, the bullet’s velocity will also increase. Certainly, the powder charge type and burn characteristics will have a major impact on how long pressure continues to increase. However, barrel length also comes into play. In a fixed volume environment, as long as the burn reaction continues, pressure will build. But in a rifle barrel, the volume available constantly increases as the bullet moves down the bore. Think of the space as the inside of a tube that gets longer and longer. At some point, the rate of volume increase will exceed the rate of pressure increase. At this point, there is nothing that is making the bullet move faster.

Second, we have to consider friction. It takes a lot of work to jam a bullet through a barrel. If you don’t believe me, try hammering one through a bore with a wooden dowel sometime. The bullet is deliberately oversize relative to the bore so that the jacket will engage the rifling and impact a spin. It also must be oversize to provide a seal for the expanding gas, so none “slips by.” Losing gas would reduce pressure and lower velocity as well.

While variables are infinite, you can assume that it takes somewhere between 220 and 410 foot-pounds of energy for an “average” .223 bullet with a weight between 55 and 60 grains to overcome friction forces and exit the barrel. Put differently, if the energy in the cartridge only offered 300 or so foot-pounds of energy, then the bullet would barely make it to the muzzle and fall to the ground.

OK, there’s more to it than that, but I want to illustrate the fact that friction is a big deal. Adding length to the barrel subjects bullets to more friction, so at some point, there will be too much friction from barrel length and velocity will start to decrease as you add more inches.

All this is a long way of saying that there are diminishing returns at play when it comes to getting more velocity out of a longer barrel. At some point along the barrel length spectrum, you’ll start to get less velocity. In reality, that point of diminishing return happens with a barrel longer than most commercially available ones.

I wanted to see how much of a difference there is between the most common lengths of AR-type rifles. To minimize variables, we decided to get our hands on samples of identically made barrels in three different lengths. To make sure everything was the same, we contacted the folks at Aero Precision and asked to borrow the following AR uppers; all chambered 5.56mm NATO.

  1. Upper Receiver: AR15 20″ 5.56 Complete Upper w/ Pinned FSB & A2 Handguard
    • Barrel: 20-inch, 5.56 NATO, Rifle Length, 1:7 Twist, 4150 Chrome Moly Vanadium, QPQ corrosion resistant finish
  2. Upper Receiver: M4E1 18″ 5.56 Rifle Length Complete Upper Receiver
    • Barrel: 18-inch, 5.56 NATO, Rifle Length, 1:7 Twist, 4150 Chrome Moly Vanadium, QPQ corrosion resistant finish
  3. Upper Receiver: 16″ 5.56 CMV Barrel w/ Pinned FSB, Mid-Length / COP M4 Upper
    • Barrel: 16-inch, 5.56 NATO, Carbine Length, 1:7 Twist, 4150 Chrome Moly Vanadium, QPQ corrosion resistant finish
  4. Upper Receiver: C.O.P. C4 Carbine Length Upper Receiver
    • Barrel: 8-inch, 300 Blackout, Pistol Length, 1:7 Twist, 4150 Chrome Moly Vanadium, QPQ corrosion resistant finish

As you can see, these Aero Precision barrels are identical except for length. They’re made in the same shop, from the same steel, using the same techniques. In case you’re wondering, they’re all button rifled.

Again to keep as much as possible constant, I used this Springfield Armory SAINT lower for all testing with all uppers.
Again to keep as much as possible constant, I used this Springfield Armory SAINT lower for all testing with all uppers.

Barrel Length Velocity Results

Using the same Springfield Armory SAINT lower, I proceeded to shoot a boatload of ammunition through each barrel to measure velocities. I set up a Shooting Chrony Beta Master Chronograph 15 feet from the muzzle and did all the shooting on the same day, so the temperature was not a significant variable.

223 velocity number in difference barrel lengths
223 velocity number in difference barrel lengths

So what can we conclude from all this? For the most part, as expected, as the barrel got shorter, the velocity decreased.

There was one anomaly that stumped me a little. Notice that the American Eagle 55-grain FMJ ammo INCREASED velocity when I moved to the shorter 18-inch barrel from the 20-inch version. I figured this was a measurement error. Since it’s practice ammo, the velocity varies more than with match ammo, so I shot a ton more. Guess what? No change in results. Average velocity was always greater, with this specific ammo, when fired from the 18-inch barrel as compared to the 20-inch one. Weird science?

I tested velocity all on the same day, using the same Shooting Chrony Beta Master setup to minimize variables.
I tested velocity all on the same day, using the same Shooting Chrony Beta Master setup to minimize variables.

There are some interesting observations if we take a closer look at the numbers. You see the following effect with all three ammo types, but let’s zero in on the Norma Match Ammo since the velocities in any rifle are consistent. When moving from a 20-inch barrel to the 18-inch, the velocity decreased by about 52 feet per second. If we assume things are linear, that equates to 25 feet per second per inch of barrel length. If you’ve heard the “rules of thumb” about velocity and barrel length, that’s about what we would expect.

Now look at the difference between the 18-inch and 16-inch barrels. Velocity decreases by nearly 100 feet per second or 50 feet per second per inch.

That’s a big change, right? It’s not a fluke as we see similarly dramatic drop offs with the other two ammo types as well.

For velocity and accuracy testing, I used a combination of everyday ammo like American Eagle 55-grain FMJ and M855 Steel Core and Norma 77-grain Match ammo.
For velocity and accuracy testing, I used a combination of everyday ammo like American Eagle 55-grain FMJ and M855 Steel Core and Norma 77-grain Match ammo.

So why does this happen? It’s almost certainly not because the “powder didn’t have enough room to burn.” Generally speaking, all the powder THAT IS GOING TO BURN does, in fact, burn in the first few inches of barrel length. What it does tell us is that the gas cloud is still expanding faster than the volume of the barrel is increasing when the bullet exits a 16-inch barrel. In other words, pressure is still building when we run out of barrel in the 16-inch rifle, so some of that pressure is wasted. There is one other variable that probably contributes to this difference.

On the 16-inch barrel, the gas port is closer to the chamber, so gas is vented earlier in the process.

Just for kicks, since I had access to an 8-inch Aero 300 Blackout short barrel, I decided to test that against a standard 16-inch barrel. I didn’t have an Aero 300 Blackout in that length, so I compared against a Daniel Defense DDM4v5 300 Blackout rifle. Both the eight and 16-inch barrels had 1:7 twist rates, although they were made by different manufacturers using different techniques. The Aero barrel is button rifled while the Daniel Defense is cold hammer forged, so understand that materials and method of manufacture might be responsible for some of the velocity variance too.

300 blackout velocity different barrel lengths

With the 125-grain supersonic load, the velocity loss going from a 16-inch barrel to an 8-inch barrel was about 255 feet per second or 31 feet per second per inch. With the subsonic load, there was only an 80 feet per second total loss or about 10 feet per second per inch.

That surprised me a little bit. If you’re going to shoot mostly subsonic ammo in a 300 Blackout, you might as well consider a short barrel.

If you're going to use a 300 Blackout primarily with subsonic ammunition, I see no reason not to use a short barrel like this 8-inch Aero Precision model. The folks at Silencer Shop lent me this Gemtech 300 Blackout suppressor for testing.
If you’re going to use a 300 Blackout primarily with subsonic ammunition, I see no reason not to use a short barrel like this 8-inch Aero Precision model. The folks at Silencer Shop lent me this Gemtech 300 Blackout suppressor for testing.

What About Barrel Length & Accuracy?

For accuracy testing, I used a concrete bench supporting a Blackhawk! Titan rest weighed down with a 25-pound bag of lead shot - plenty stable!
For accuracy testing, I used a concrete bench supporting a Blackhawk! Titan Benchrest weighed down with a 25-pound bag of lead shot – plenty stable!

While I was at it, I decided to do some accuracy comparisons. I set up targets at 100 yards and proceeded to shoot a ton of five-shot groups with the American Eagle M885 and Norma 77-grain Match ammo. I used the same Burris XTR II 2-10x Optic and moved it to each upper in turn. I shot from a Blackhawk! Titan rest weighed down with a bag of lead shot and used a rear bag for stock support. All this gave me a solid shooting platform which removed as much shooter error as possible. After shooting multiple five-shot groups with each combination of barrel and ammo, I averaged the group diameters to see whether barrel length had any impact on accuracy. Since I was running low on the American Eagle 55-grain FMJ, I limited my accuracy testing to the M855 and Norma Match.

223 Accuracy different barrel length

Theories on the results? Looking at the M855, I could guess that the longer barrel MIGHT have stabilized a less than perfect round a little better. Admittedly, that’s a weak theory. Typically if the twist rate is right to stabilize a bullet, extra barrel length doesn’t really matter. If you forced me to explain the progressive decrease in accuracy of the M855 as the barrel got shorter, I’d bet a maple-glazed donut that it’s just ammo variance. I’ve not found M855 to be very consistent in any rifle, so odds are pretty good we’re just seeing some randomness.

For accuracy testing, I moved this Burris XTR II 2-10x scope to each upper as required.
For accuracy testing, I moved this Burris XTR II 2-10x Scope to each upper as required.

As for the Norma 77-grain Match ammo, note how accuracy of the barrels at the two extremes are almost identical. That is interesting, and supports the theory that once a bullet is stabilized in the first few inches, the rest doesn’t really matter. As for the 18” barrel and it’s better accuracy? I’d guess that particular one is just a slightly smoother and polished bore than the other two.

To me, the big learning is that the 16-inch barrel, with a whopping four inches shorter length, was just as accurate as the 20-inch version when using quality match ammunition.

Barrel Length: So What’s The Bottom Line?

I’m not the only one to observe that extra barrel length really doesn’t improve accuracy on its own. If you’re after a super-accurate rifle, get the length you prefer for handling and spend the money on barrel quality.

As for velocity, here’s how I might approach the situation. Rather than pondering raw velocity numbers, think about your application. If you’re hunting two or four-legged critters, you might want to consider the range at which you need your bullet to be effective and work backward from there. Here’s what I mean by that.

Say you want 500 foot-pounds of energy at the target, and you’re shooting that 62-grain M855 projectile. When you use the 20-inch barrel, the muzzle velocity will ensure that you have 500-foot-pounds to just past 415 yards. If you shoot the same bullet from a 16-inch barrel, you’ll all below 500 foot-pounds at 375 yards. Or you could look at the point where the bullet crosses the supersonic threshold and starts to lose accuracy. For the 20-inch barrel, that will happen at 830 yards. For the 16-inch barrel, that happens at 775 yards. OK, not a huge difference either way, but that’s how those differences in muzzle velocity translate for energy and supersonic performance down range.

Those down range performance differences aren’t all that dramatic, so unless you have very specific needs, get the rifle you like, regardless of barrel length. In most scenarios, it’ll do what you need.


Tom McHale is the author of the Insanely Practical Guides book series that guides new and experienced shooters alike in a fun, approachable, and practical way. His books are available in print and eBook format on Amazon. You can also find him on Google+, Facebook, Twitter and Pinterest.

  • 33 thoughts on “AR-15 Rifle Barrel Length – Does It Even Matter? Maybe NOT

    1. it’s also interesting that you got lower muzzle velocity out of the AE 55gr ammo than the mftr claims it will produce (by 2-300fps)

      1. Ammunition manufacturers typically test ammunition in a somewhat custom setup, typically a single shot or bolt action rifle with a 22″ or 24″ barrel. A significant velocity difference would be expected when moving to a 20″ or shorter semi-auto rifle.

        As to the article’s premise, barrel length is a lot like barrel twist rate. It will matter if you’re trying to get the best performance out of a specific ammunition choice, but if you really only care if the rifle will fire and function reliably then it really doesn’t matter for you.

    2. .22 LR generally has the highest velocity in a 16 inch barrel because the small powder volume is completely burned in 6 inches and it soon stops expanding.
      Remington used a 27-1/4 inch barrel on the 40 XB .22 target rifle because the reduced muzzle pressure does not disturb the bullet as much as it leaves the barrel. Most AR have some sort of muzzle device which may reduce muzzle blast bullet disruption and improve accuracy.

    3. If you want to hit at 400 meters or greater use a 20 inch Barrel!!!!!!!!!!!!! The M4 has a 16 inch Barrel just for urban warfare! Anything shorter, get a Damn Pistol! An AR-15 Pistol is Just STUPID! It’s Bulky and will only get you Killed! The Purpose of a Pistol is to get you a Rifle!!!!!!!!!!!!!!!!!

      1. first a m4 has 14.5 inch barrel, ar pistols are commonly used in the same way as a rifle just without touching it to your shoulder…kinda like a sbr except its a pistol

    4. If you are shooting .22 caliber/5.56mm (and when did real Americans start using metric units?) you are about two (2) calibers shy of a REAL rifle – not too fond of the .308 Winchester – but the 30-06 just, “fits my pistol – er, rifle”. Doesn’t do you much good to, “kill your target”, if you do not stop it before it gets to you! Why do you think that Uncle Sam went from the .38 Long Colt to the .45 ACP?

      1. By the way, the US Department of Defense uses the metric system almost exclusively. Most of them are real Americans, last I checked. And physicists the world over use it. And engineers! The Army Corps of Engineers!

        And it is a better measurement system …

        1. Because you say? Arrogance over competence. Oh and DOD switched “by order of” not because some Europuke says it’s a better system or because they decided unilaterally. I t was/ is all part of the cumbya one world BS from our NON-servants of the PEOPLE. So it’s about money. As always.

          Our system worked well enough to get WE THE PEOPLE through a couple WWs!

          1. You are right Patriotism-Matters, our bean counters were attempting to standardize the purchase of ammo for the one world order. Most are not aware that 9mm is 38 cal. and that 380 is just a 9mm Short. After awhile, they realized that 9mm drilled holes in the air and bodies and was not real good at stopping people unless you hit them in the right spot. Even with all of that realization that the 45 cal. was/is a good stopper, it appears that politicians and bean counters will prevail again with the 9mm. Oh the frustration of the poor warrior out in the field fighting for his life. Of course the best thing to do is buy your own work tools.

          2. If you check the trend over the last 100 years, countries switch to the metric system from whatever they were using. Once on metric, nobody changes to a different system.

            Our system is called the English or Imperial system. We’re not English, the English use the metric system and still will after Brexit, we’re not an empire and we have t been part of the British Empire since the Treaty of Paris.

            Moreover, it’s just a measurement system, not a prof of manhood nor a loyalty test.

            And .45 caliber is 11.5mm to me. Awesome.

            1. Whaa?! Not a reflection on my manhood?! heheheh
              But it is a royal PITA for us gheezers that grew up on the Imperial system.
              So… your comment about .45/11.50,mm is timely since I’ve just uncovered a box of 11.43 ammo (french>?) that I assumed was .45
              Were you rounding up to 11.5 from 11.43? Or, would you know, do I have some other animal?

    5. Agree with Roy, the AR pistol is a stupid idea. It is combersom and very hard to conceal. Get a real quality pistol and cut the weight in half and you’ll be better able to hit your target. The AR pistol is a gang gun because most of them no nothing about guns but think they have something “bad”.
      As far as the accuracy test goes, I’d pick the 18 inch. It produced great accuracy and isn’t so long to make it cumbersome. I have an Armalite AR with an 18 inch barrel and it is the most accurate rifle I own. My two cents are now spent.
      Thanks for the interesting test.

      1. “The AR pistol is a gang gun…”

        Having retired from LE and working with gangs and working with other states agencies that work with gangs I’d like to know where you are pulling that data set from. Short-barrel ARs (pistols or legal/illegal SBRs) were never an ‘issue’, or a reported issue with the various groups I interacted with. AK variants, including short-barreled or pistol-pattern AKs were scattered here and there, but AR pistols? No.

      2. not accurate, Roy & Eric :
        Check out the Rock River Arms LAR-PDS
        no buffer tube to contend with and will shoot multi.
        The perfect “Truck Gun” in rifle caliber.
        Throw a micro red dot on it and viola’ — very concealable and steathy
        No need for some silly SBR – let alone the tax stamp craziness…

    6. 20″ provides best ballistics from 3′ to 300 meters…. its short enough for cqb……. so get iver it 20″ was right the first time….

    7. This is a great article. 77 gr bullet hates extra bore time. Stress on its body length best kept breif. Hence better accuracy in shorter bore. 1/7 twist usually slowes em down a bit aswell. Crony with a 1/ 9 u will c velocity increase with the appropriate weight bullets. Accuracy was pore, for all loads but i wasnt shooting. And 16 inch barrel may shoot as accurate but wont keep cheep body bullets spinning true past 650. A 20 inch barrel will hold true till 850 atleast! Try it yourself. And this is the short and sweet of a wholr article myself.

    8. All this means NOTHING without taking into consideration the burn rate of the powder used. A faster powder will make a difference in a shorter barrel simply due to the fact that it doesn’t need the extra length of the barrel to complete combustion. See where I’m going here? So if I used a slower powder in the longer barrel and faster powder in the shorter, it could be possible to get the same velocity from the two.

      1. This is a myth. All the powder that’s going to burn does usually burn in the first couple of inches. Muzzle flash is mostly incandescent ash, not burning powder.

    9. Haha~Thats were we get into a whole article, Lol, length wise I mean. Also yes-bullet types, body shape, quality control, seating depth, and powders. To refresh, Imho, typically most real world shooting with such weapons and barrel lengths involves factory or military loadings and those powders usually vary only by lot once chosen for use and hence the barrel is next up. but i do Another thing to keep in mind factory and military powders tend to unavilable to comsumers.

    10. On the other hand cheep body bullets are readily available to us consumers! And like to destabilize at long range as they slow down! Thus a 20 inch or 24 inch barrel can really surprise those who havent spent the time throwing long shots( past 600 atleast) in an AR-15 platform

      1. This is the Viscount’s wife writing. I’ve snagged his iPhone whilst he golfs.

        I have a bit of experience with a wide variety of rifle barrels, none other than the Viscount’s since our nuptials of course, and for my trigger finger a 6″ — err, 16″ barrel of sufficient girth is my choice for daily shooting.

        Now, who wouldn’t enjoy laying down under the stars on a lovely patch of clover with a 24″ barreled AR-15 for an evening’s worth of champagne and target practice? But for day-in day-out, 16″ works great with less clean up.

        And in any close quarters situation, don’t back me up with a 24″ barrel. It’s a bit unwieldy, young man. You will spoil a good hunt if I catch you pointing that thing at the upstairs bathroom instead of the downstairs play room where the target is.

        His Lordship returns from the links and will be wanting his tea …

    11. The AR pistol has a place despite what some are saying here. It shouldn’t be your first choice its a choice of convenience for professionals and people who need concealment. Many have no idea how to actually shoot it though. Not their fault its just that we on this side of the pond haven’t been trained and social media isn’t helping.Its a much better option in many cases to a 9mm subgun.
      Also if it is an emergency use what the ATF says you can’t to stay alive and worry about details later.
      No matter what you carry though test your equipment and loads and do not go by what anyone says or writes (including me).

      1. I have several ars, and a pistol, 10 inch barrel that has a specific purpose and place. It is only for the no knock midnight raids, or for breach and enter roon clearing. I have a fancy trigger that comes close to full auto, and it has a 100 rnd drum. My version of an mp-5 on steroids. Certainly works in a situation where you need immediate overwhelming firepower. My other ars have 16 inch barrel, 1:9 twist and I run 55 gr to 62 gr and don’t intend for it to reach out any further than 400 yrds. Any hit that spans from shoulder to shoulder chest to waist, as far as I am concerned, does the job. Any further need for any greater accuracy, I will pull out either the AR 10 set up for sniper work, or the M1A, also so set up. I have another AR 10 that is also 16 inch barrel, and pure tactical. Bottom line, each barrel length has a purpose and place. Know what you want your tools to do, know the strengths and limitation and build accordingly. If you ain’t gonna use it for sniper or match work, then none of this really makes a profound difference

    12. No criticism of the shooter, however, is there any explanation as to why the 16 in barrel could barely manage a sub-5 inch group at 100 yds? Especially considering the stability of the shooting platform and the optic used? After all, Aero precision is mighty proud of their barrels…

      1. As the article clearly states, M855 steel core is amazingly inconsistent when it comes to accuracy. Same rifles with different ammo yielded 1.5″ groups if you’ll look at the results.

    13. 1. The variables are many but not infinite.

      2. Shocking lack of proof-reading, folks.

      3. Another lesson that quality of construction and skill of the user is more important than mere length. My wife points out that barrel girth is also a factor, for accuracy and varmints.

      4. Before he was “Mad Dog” Mattis, he was “Two Left Feet” Mattis. At last at the Academy. Really was uncomfortable on the debutante cotillion circuit. Oh the stories…

    14. A very interesting and informative article. Tom, I’d invite you to look at a related but different scenario. There are now many ARs chambered in pistol calibers. And there have been many tests in handguns of those calibers, but I haven’t seen a lot in longer platforms such as ARs. I have been told by an armorer that a rule of thumb is that you can add 50-60 FPS to handgun cartridge performance in an AR out to 16 inches of barrel length. I’d like to see that rule of thumb tested.

    15. OK, so now the debate has been settled on barrel length. It all fails to establish the 5.56mm as a consistent man stopper compared to the 7.62mm or the 30/06. Me being an old Corps Marine, carried an M14 in RVN until reissued an M16 with 1 month left in my tour. Only benefit to be derived from the M16 was being able to carry more ammo which for a rifleman only makes sense when the lighter ammo is as effective as the heavier stuff. My M14 always worked where the M16 would not empty a magazine without a FTF or a FEE, both deadly in Indian country.
      The whole debate of an AR being a great house gun is flawed when muzzle blast and over penetration are taken into consideration. Yep, I am a pistol guy for most self defense and an AR for longer range in populated areas due to less over penetration compared to any .30 cal.

    Leave a Reply

    Your email address will not be published. Required fields are marked *