As far as AR15 components go, your barrel length holds one of the most critical roles in an AR platform. The barrel length directly affects your ballistic coefficient and it’s important to consider factors for legal compliance. The answer to what barrel length for an AR15 is suitable for you in terms of performance depends on what you want to do with it. Are you looking for a precision rifle to make tight groupings out to 400+ yards? Or perhaps you’re looking for something to engage targets at a lesser range and in close quarters? This is our guide to give you some pointers in the right direction for your AR15 build.

How Does Barrel Length Affect Velocity?

Greater velocity means more energy for the projectile to potentially transfer into your target. This is what terminal ballistics is all about. Imagine your bullet is an airplane and you can think of the barrel as the runway for said plane. The longer the runway, the more time it has to spin up for speed. The more speed a projectile has, the flatter its flight path is, kind of like a football. What’s technically happening is that the gas expansion of the propellant’s combustion gets more time to do its job of sending a projectile down the barrel. As you can imagine, the more force exerted onto the bullet is what provides greater velocity, an effect that is increased with longer barrel lengths.

AR 15 barrel length accuracy is not solely affected directly by a barrel’s length. The rifling plays a huge factor as well. However, the combination of the type of barrel you choose along with what kind of rounds are shot through it will determine whether or not the barrel is not long enough for the powder to finish burning up before the projectile leaves the barrel. If the barrel is too short, then you won’t be getting optimal performance out of your rounds.

There are some calibers that are designed to burn quickly and ensure that you get the most performance out of a short barrel though. 300 Blackout is a great example for a caliber that is designed specifically to achieve its best ballistic performance out of a 9” barrel length as talked about previously in our 300 Blackout Complete Guide.

AR15 Gas System

Gas blocks are the regulators of how much gas is diverted from the barrel to the gas key sitting on the bolt carrier group (BCG). Too little gas and there is not enough force for the BCG to be pushed back for the AR to properly eject the spent casing and chamber the fresh round. On the other hand, too much gas means a more violent recoil albeit your AR is still going to cycle. An overgassed AR tends to easily get filthy as excessive carbon builds up in and around the firearm parts. With more violent recoil, components facing unnecessary or extra wear can quickly whittle down their life span.

Some of the other issues that can be encountered with overgassed AR rifles include insufficient timing of the bolt locking mechanism, failures to eject, and failures to feed. Any AR with a poor gas system is an unreliable AR. Fine-tuning your AR gas system is all about finding the right amount of gas to consistently and reliably cycle the BCG back and forth. This can be done with an adjustable gas block that can set exactly how much gas is allowed to enter the gas tube. Conversely, you can also use a fixed gas block with an adjustable gas tube. There’s a lot of options out there today.

What gas tube length do I choose for my AR15?

AR gas systems are broken down into four different lengths, each starting from the barrel’s gas block port and ending embedded within the upper receiver. The gas system length is dependent on the AR barrel length so make sure you buy the correct one per your build.

AR gas tubes come in:

• Pistol length - 4” gas tube. Suitable for barrels less than 10”.
• Carbine length - 7” gas tube. Suitable for 10” through 18” barrel
• Mid length - 9” gas tube. Suitable for 14” through 18” barrel
• Rifle length - 12” gas tube. Suitable for 20” barrel and longer.

Do Barrel Lengths Affect Longevity?

The length of the barrel does not have a noticeable impact on the barrel’s longevity, although a longer barrel may be better at withstanding the punishing heat from rapid fire.

Continuous heavy firing schedules have the most impact on your barrel longevity. Allowing your barrel to reach high enough temperatures from continuous fire, 900 to 1200 Fahrenheight, makes it vulnerable to excessive wear. Essentially, avoid mag dumping too often. Let your barrel cool down in between during your range sessions regularly will greatly prolong your barrel’s life.

The lifespan of a barrel can also be affected by the ammo you feed it and as mentioned before, how you shoot it. An average barrel that sees at least minimal maintenance and isn’t being treated like a throwaway tool can be expected to last somewhere between 10,000 to 20,000 rounds. If a high quality AR 15 barrel sees the right care and a diet of non-steel ammunition, you should expect a lifespan of anywhere between 30,000 to 50,000 rounds before it gets shot out.

An AR barrel that has been “shot out” will show a bore that has been smoothed out with faint markings of what used to be the rifling. Stainless steel barrels are typically more accurate than that of a cold hammered forged chrome lined (CHF CL) barrel. However, the latter is built to outlast the stainless steel match barrel every time. Shot out barrels can also be identified when reviewing your shots on paper or cardboard targets. If you find “keyholes” then you know your barrel already has at least one foot on the stairway to heaven.

What Barrel Length Works Best For Home Defense?

For urban engagements and home defense, an 11.5 inches is a great AR 15 barrel length for those purposes. It’s popular for several reasons. While the AR15 is a host to many calibers, .223 Remington and 5.56 NATO are essentially the most popular rounds to use for it which even despite the ammo crisis during the pandemic, they're still widely available for purchase online.

Up against soft targets, an 11.5” AR 15 barrel would still allow you to have great terminal ballistics which involve rounds delivering the optimal expansion of body tissue and creation of wound cavities that would ensure stopping a hostile individual.

Going shorter than even an 11.5” AR15 barrel may offer you better maneuverability and reduced weight, but you sacrifice a great deal of ballistic performance with those few inches of barrel length reduction. At 7.5” barrel length, you might as well use an AR-9 instead because the terminal ballistics of 5.56 NATO out of that short of a barrel are comparable to shooting 9mm at that point.

Why you want to avoid longer barrels

Obviously, longer barrels are not a good choice for home defense. There are a couple of reasons for this: Maneuverability and over-penetration. Whether it’s in the hallways of your home or your vehicle door, longer AR15 barrels can prove to be cumbersome for weapon manipulation. Opening doors and doing anything with two hands while your 16” - 20” barrel length rifle dangles off of your body is something you don’t want to deal with in a high stress setting where making decisions quickly can make all the difference to you and your family’s safety.Over penetration is something you must consider for home defense. How it relates to barrel length is because if you use an AR15 with too long of a barrel in conjunction with a caliber that just goes through multiple barriers before stopping then you’re only creating an even bigger liability issue for yourself because the longer the barrel, the faster that projectile will travel.

One advantage 5.56 NATO, M193 ball has over its steel penetrating, tipped brother (M855 ball), the M193 will tumble and break apart upon first contact with even drywall. For further reading on the nuances of M193 and M855, check out our other blog, M193 vs M855, on this matter.

What is the Ideal Barrel Length For AR9?

Unlike rifle calibers, a pistol caliber AR does not need a long barrel for the round to reach near its fullest potential. Some may notice that a heavier weight, such as that from a longer barrel, slightly reduces the recoil and helps with follow up shots. Conversely however, most shooters prefer the compact light package of a pistol caliber carbine and can be satisfied with the ballistic performance of an 8” barrel.

In one of our previous blogs, What Is An AR9?, we covered the differences and similarities of an AR9 to an AR15. Be that as it may, there are actually plenty of competitive shooters who have full length PCC builds that shoot AR9s with 15 or 16 inch length barrels.

What is the Legal Length of an AR-15 Barrel?

In 1960, the National Firearms Act of 1934 was amended to restrict the legal barrel length of centerfire rifles from 18” to 16”. If you’re wondering how to measure AR 15 barrel length, begin from the barrel’s breech to the end of the muzzle. If you have any muzzle devices such as a faux suppressor, flash hider, muzzle brake, or compensator affixed permanently to the barrel it will count as a part of the barrel’s length too. This leaves your options open to 14.5” barrels or even 13.9” AR 15 barrels provided that you attach a muzzle device to the barrel as how ATF defines permanent and creates an overall barrel length of 16” or longer:

• full-fusion gas or electric steel-seam welding,
• high-temperature (1100°F) silver soldering
• blind pinning with the pinhead welded over

To be frank, it would make no difference in how you do this so long as you use a method that makes it so that an ATF field agent is unable to remove the muzzle device from your barrel. At the end of the day, is it really that likely for anyone to be running compliance checks in the field or at the range with a vice, armorer’s wrench or a torch handy.

Subsequently, if you’d like to have your rifle under the 16” barrel length or an overall weapon length of less than 26” then you must file a Form 1 with the ATF to buy an SBR tax stamp.

What are the Advantages of a 16” AR Barrel Length?

A 16” AR 15 barrel length for a .223 Wylde barrel would have enough to push a velocity just slightly under 3,000 fps. This is typically suitable for most first time gun owners’ purposes and has the benefit of shooting softer than a 20” barrel. Although the length is not ideal for close quarters and general maneuverability in environments such as enclosed hallways, a 16” AR 15 barrel length has enough real estate for you to fit a handstop in the spot exactly where you want it to be with room left over for weapon lights, illuminators, and frontal iron sights. Additionally, when it comes to aftermarket support the 16” barrel length is one of the most common AR 15 barrel lengths in prebuilt upper receivers.

• Legal Compliance unless you have a $200 Short Barreled Rifle (SBR) Tax Stamp and aren’t in California
• Can swap muzzle devices freely without going under the 16” barrel length minimum requirement
• A compromise between velocity and weight
• Has plenty of rail space for accessories
• Most easy AR 15 barrel length to find handguards for
• Heats up slower compared to shorter barrels

Haven’t Decided On Your Barrel Length? Start With Your Lower Today!

Regardless of what AR 15 barrel length you decide on, our Router Jig Extreme and selection of 80 lowers can be used to produce an AR lower receiver suitable to whatever caliber you’d like. Our jig is suitable for repeated use to continue to assist you in making as many lowers as you could need with no experience in milling needed. Feel free to contact our customer support for any of your questions relevant to our rigs.