How to Determine the Right AR-15 Barrel Length for Your Build
If you have been in the AR-15 market for about five or six minutes you’ll quickly notice that we have a difficult time agreeing on things. One of the most important ones is barrel length. Some of those who have been around since 2004, when the AWB expired, will have seen the whole barrel length roller coaster and many of the length wars inspired by each generation of barrels.
So which barrel is best? What do you want and what do you need? What should you avoid? Let’s tackle these and some of the myths surrounding them.
“Flash Bangs” – Barrels Under 10.3“
Let’s tackle our first myth, “Short barrels are less accurate.” This is patently false. The accuracy of any barrel is a combination of the quality of the barrel, the quality of the assembly, and the quality of the ammunition you are shooting. Your barrel and your ammo do determine one very critical thing for making your shot though, and not understanding it will assure that you miss just as certainly as an inaccurate gun. Muzzle velocity. How quick that round leaves the barrel and starts suffering the most ill effects of wind and gravity is directly tied to muzzle velocity.
Short barrels produce the slowest muzzle velocities. The slower muzzle velocities result in the most rapid drops over distance and are most quickly subject to the wind. The round will also reach transonic, where it passes back through the sound barrier from faster than to slower than the speed of sound, and that will also disrupt the flight of the bullet.
So short barrels aren’t less accurate than longer ones, but they have a shorter distance to work within before the environmental factors start moving that shot around a lot and you have to really start doing the math to predict where the shot will land.
Now, why call them “Flash Bangs” in the section heading? Barrels that are this short are both very loud and produce a tremendous amount of muzzle flash, simple answer. The reason for that is the powder charges are optimized to burn in a barrel that is longer. The original 5.56 and .223 rounds were designed and charged to burn in 20 inches of barrel, 8.5 inches is significantly less than that.
Think of it like a car accelerating. If your car’s engine can go zero to sixty miles per hour in 4.6 seconds you need all 4.6 seconds to reach sixty miles per hour. Think of the barrel length like acceleration time, if you only have 2 seconds instead of 4.6 to accelerate you aren’t going to hit sixty miles per hour, but you might make it up to forty miles per hour.
Now all the unburnt powder that would be making gas and pushing the bullet instead gets tossed into the air at the muzzle as the bullet leaves. It burns nice and bright like a Michael Bay movie. But it’s all flash and noise, it doesn’t help the bullet anymore.
We had a period at the turn of the 2010’s where super short barrels were all the rage. Small subcompact carbines were the rage. Who cared if they worked optimally, they were small. Many companies made them work well even, still do. You can find rifles with barrels in the 8.5, 7.5, even 5.5” and they work. But they don’t outrun physics and a bunch of powder in a 5.56 or .223 round is going to turn into light and noise instead of speed.
Should You Buy a “Flash Bang” Barrel?
If you value the compactness of the carbine most then this might be the answer for you. Make sure it is gassed and buffered properly when it’s all said and done. There are suppressors that can tame the noise and flash down too and still maintain a small handy carbine.
Don’t expect to reach out to the distances even slightly longer guns can though, that muzzle velocity limit makes it a challenge and you’ve bled off easy distance and energy if you’re hitting a target you need energy. But inside that 100 yard envelope they are quick, handy, and fun.
“MK18 Bro” 10.3” Barrels
This length exists in the commercial space entirely for the purpose of emulating the military MK18 CQBR. The 10.3” uppers the military originally built to have a suppressed carbine in the same space profile as the M4.
Why 10.3” long? They couldn’t chop them shorter and make them work how they wanted. If you look into the 10.3’s, they’re persnickety on their gassing and buffering but they do work. They were built to run military 5.56 ammunition with specific suppressors, so using commercial off the shelf stuff in different pressure ranges doesn’t always play nice. But if you want the original CQBR size, you are getting a 10.3 inch. At Aero Precision, we rounded this to a 10.5 inch barrel for your AR.
Should You buy/be a CQBR MK18 Bro?
If you want. They were a very effective fighting carbine in a time when they were needed, but like many things from the GWOT era we’ve moved onto newer, more popular systems.
“Best” AR15 Barrels (of the moment) 11.5” and 12.5” Barrels
Here we finally hit the current, and usually most agreed upon “Best EVER” barrel range for the AR-15 rifle series. Short enough to be very handy. Long enough to get the velocity up and not be problematic with gassing and cycling the gun.
You can call this the ‘Goldilocks’ range, they’re just right. The 11.5” barrel is the updated CQBR standard and numerous agencies and professionals are using the 11-13” barrel length range for their guns.
One significant downside to these lengths, and all the shorter lengths, is they’re NFA controlled rifles. They all make SBRs under the current ruleset and need the proper stamps to be built or bought. If you don’t care about the stamp cost and wait times though these easily make for the most mobile modern general use carbines on the market and are the current professional standard.
Side Note: We are not talking about twist rates or coatings much because they aren’t going to make or break the guns. Here’s the short version. Rifling twist rates of 1:7 or 1:8 are fine. chrome lined or nitride or stainless barrels are fine. Cold hammer forged rifling or regular button cut rifling are fine. You can deep dive the specific benefits of each of those and see if they fit your use, but don’t get too wrapped up in any particular difference. Shooting 55gr bulk target ammo will work in all of the above and produce good results. Stop worrying. Shoot more.
Should You Buy an 11.5” or 12.5” AR Barrel?
If this is your first AR15 build; looking for versatility; or you’re not planning on doing a variety of AR-builds – then yeah, this is a common choice by many.
“Barely Legal” 13.7”, 14.5”, and 14.7” Barrels
Why 14.5” barrels? The M4 and M4A1 use 14.5” barrels and are the general issue service carbine of the US Armed Forces. It is the standard. We know how to build them. We load ammo for them. We’re operating these carbines back in the original range of what 5.56 rifles were built around.
Why 13.7” and 14.7” barrels? Because they both have common associated muzzle devices that are easy to permanently attach to the barrel and make them the minimum legal length on a rifle to no longer be considered NFA controlled firearms. They are the “barely legal” barrel lengths that make them Title I firearms with a permanent muzzle and the least legal hassle to purchase, possess, transport, etc.
These also keep your carbine as short as it can be and remain Title I, usually with a very good quality suppressor option for whichever muzzle you permanently attached to the barrel.
Side Note: Don’t worry about not being able to swap muzzles later on “permanently” attached muzzles. All that a pin & weld does is bind it to the threads so it can’t be twisted off by hand or a wrench. An armorer with the right tools can swap muzzles without too much effort if you want to make a change later to accommodate different accessories. Permanently attached is in the legal sense, not the physical.
Should You Buy 13.7”, 14.5”, and 14.7” AR Barrels?
This barrel length range makes for very handy and well rounded carbines. They have more velocity at the muzzle and can reach out further. All these guns can benefit from shooting the heavier match ammo in them and seeing the results, but the results tend to be more dramatic as we get into these longer barrels. More on that in a bit.
“RECCE!” 15.1” Barrels
There’s really only one reason you’d be looking at this barrel length. You want to clone the SEAL/NSW carbine from one specific accurized version. You think it’s cool.
That is it. That is the reason. You are less than an inch from the completely standard, easy, simple, Title I 16” barrel lengths, and various RECCE and SPR rifles went all the way to 18” barrels.
RECCE carbines aren’t limited to 15.1” barrels, with the exception of one specific build style that used those barrels. It is cool but, any attempt to accurize an M4 and give it more reach without lengthening it significantly can be considered a ‘RECCE’ build.
“Should I buy a…?”
Yes, you should buy it if you want it or are interested in them. They will perform and even with their legal quirks and hoops they’re fun to rock and roll with. Just make sure you, if you’re building, build with the right parts to make the run. Don’t cheap out on a part to save eleven dollars and then the buffer is too light or the gas system isn’t feeding the right amount to the action to run it. It’s a machine, give it the correct parts to run correctly.
Back to barrels.
“Sweet 16” 16” Barrels
Here is your simple, least hassle, most performance for shortest rifle, off the shelf option. The 16” AR-15 barrel is the commercial industry standard that the 14.5” M4 is for the military. Thanks to the NFA they’re different. It would probably be 14.5” if not for that, but the 16” gun is a workhorse in its own right.
Should You Buy a 16″ AR15 Barrel?
I might be bias, but this is my favorite length. So when in doubt about which AR barrel length you should buy, then get this barrel and you won’t have made a bad decision.
“MK12 SPR” 18” Barrels
The MK12 military rifle really opened the door to the wider public looking at the AR-15 as capable of precision, and the MK12 has an 18” barrel. Long enough that you really are getting optimum powder burn from your ammunition, especially tuned match ammunition, and suddenly everything with 700 yards can be touched with a shot if you do your part behind the rifle.
Your trade off from shorter options is just that, the rifle isn’t short anymore. We are away from the ‘carbine’ and into rifles now. You can keep these guns fairly light weight still and with modern suppressors they still aren’t so long they are obnoxious, but if you are building them out to take advantage of that higher muzzle velocity and longer reach than the whole package of accessories are going to add weight.
Don’t fret, heavier makes them easier to keep steady and a steady rifle sends a steady shot. I might want a 6-8lb rifle with a dot on it for point and shoot shots inside 200 yards, but a 12lb and well scoped AR-15 that’s sitting still in the hand for a 600 yard shot is just what the situation requires.
The MK12’s 18” barrel helped define this genre of AR-15 and there is no sign that the 18” precision 5.56 gun will lack a fan base anytime soon. You are also more likely to see rifle twists tuned for specific 5.56 projectile weights instead of the more general approach 1:7 and 1:8 rates chase after. The 18” barreled gun is much more likely to be asked for precision and the parts availability reflects that request.
Should You Buy an 18″ AR Barrel?
If you’re among those buyers looking for that performance, 18” barrels might be for you. Ask your gunsmith or armorer about SPR today!
“The Musket” 20” Barrels
We’ve reached the end of the list. Yes longer 5.56 barrels exist, but we are not venturing there in this article.
The 20” barrel is the backbone of the AR-15. The M16 and the ammo were giving the best performance jointly in this combination. The M16 kept the 20” barrel from inception through all four ‘A’ series system improvements. The M16A4 is still an incredibly capable rifle.
If you’re wondering why the military moved away? Size. Length specifically. Even adding an adjustable stock, a 20” barrelled rifle is long. It was determined that shortening the rifle, especially with the US Military’s largely mechanized (vehicle use) nature, was the answer and the M4 and M4A1 carbines would be fine. Largely they were correct.
Physics is no liar though. The 20” barrel still propels the round faster, and therefore further, with less drop or wind deflection over the distance than the shorter options. Less deviation from the point of aim and the longest non-specialized effective range are the hallmarks of the 20” barrelled AR-15. The 20” barrel series have also benefited from the accuracy experiments on the AR-15 and the results are some truly spectacular performing rifles with match ammunition once you float the barrel, something the M16 series never got around to doing.
Which AR Barrel Should You Buy?
The one that fits your primary use case. The AR-15 is a spectacularly flexible platform and just because you picked the 18” or 20” barrel for range and accuracy doesn’t mean you cannot clear a room with it in CQB or a close-in action shooting match. But build for the thing you want the rifle to do the best, then let it do the rest.
Which barrel are you buying, or which lengths do you have and why? Let us know in the comments!