What is the best barrel length for 5.56?

One of the many exceptional things about my job is everyone at CMG and RE Factor Tactical is a Military Veteran.

We’ve all been fortunate to be around ARs for decades and remain well-placed in the gun community.

There always seems to be a straight-line argument when it comes to AR barrel length and I’d like to offer a fresher perspective on AR-15 barrel length.

What is the best barrel length for 5.56? How short can you go?
Photo: Maxim Defense Industries

We all want to know which barrel length is the best for us, but the purpose of your AR is something you should settle on before you choose your barrel length.

If you’re wanting it as a home defense gun, the 5.5"-11" barrel length would be better than the traditional 16" barrel that we’re all used to.

Whereas, if you’re looking for a rifle that can shoot accurately to 500 yards and further, a 16"-20" barrel would be your best bet. 

Maxim Defense MD11

$3,895.00
at Maxim Defense
Prices accurate at time of writing

Why Would I Need A Shorter Barrel on My AR-15?

Anytime I hold an SBR or AR-pistol platform, the weight difference in a “chad" AR with a 16" barrel is incredible. Right behind the difference in weight is the maneuverability that a 5.5" barrel provides, that physics won’t allow for in a 16" barrel.

I’ve always stated my home-defense gun is a Mossberg 12 gauge loaded with 00 Buckshot, but the PDX offers more maneuverability than my Mossberg 500 and the power of 5.56mm, 7.62mm x 39mm, or a .300 BLK bullet. 

A gun like the Maxim Defense PDX is only 18.75" and can be chambered in the same caliber as any other AR at twice the PDX’s overall length. The PDX is absolutely perfect for close-quarters combat (CQB), such as urban fighting or a home invasion. 

However, I wouldn’t want to shoot at 500 yards with it versus an AR with a 20" barrel. My point is a 5.5" barrel might be for you, but it might not be if you’re not using it for CQB purposes.

What’s The Bore Pressure for an AR-15?

They measure bore pressure in Pounds Per Square Inch (PSI) because it’s the easiest and most effective way to measure the pressure coming out of a barrel.

When you choose an AR platform, knowing the PSI is a helpful way to understand how high the pressure in your barrel is. A 20" barrel has almost less than 5X the pressure of a 5.5" barrel. This is because the bullet has more time in the barrel to balance the pressure. 

Understanding the bore pressure inside each AR barrel length will also help you get an idea of the felt- recoil you can expect. A shorter barrel can have a helluva kick if shooting something as powerful as a 5.56mm or 7.62mm bullet.

There are ways that some companies, like Maxim Defense, can help mitigate the felt-recoil to allow for more shot-to-shot accuracy. My favorite feature for the PDX is the SCW stock because of the customization and an incredibly low-profile feel.

What’s The Bullet Velocity of AR-15 Barrels?

I love using a table because it shows the connection between bullet velocity and bore pressure. You can (mostly) assume that if the bullet is slower, it’s accompanied by more pressure.

For example, the fastest bullets are those with the least bore pressure whereas, the slowest bullets coincide with the high bore pressure. 

 Barrel Length Bullet Velocity (ft/s)Pressure (PSI)Max Effective Range (Estimate)
5.5"1,78024,000150-200 yards
7"2,33017,040250 yards
9"2,40013,565300 yards
11"2,57010,540350 yards
13"2,6458,885400 yards
16"2,8007,430500 yards
20"2,9755,717700-800 yards

How Short Can You Go With Your AR Barrel?

What are you using the AR for? do you need to reach targets out past 200 yards? If not, there really is no need to get a 16"-20" barrel. I’m not an OEF veteran, but I know from my time in Iraq, most of the fighting was done with 200 yards.

Obviously, I’m not talking about the potshots snipers take as we drive by, but the actual fighting was much closer, versus much of the fighting I’ve heard about in Afghanistan. Regardless of where our next wars take place, there’s no argument that an AR platform with a 5.5" or 7" barrel has its place when the gun is built right. 

If you’re interested in learning more about the legality of an SBR or AR pistol, read How to Build Your First SBR Legally.

The ability to fight in close quarters within urban environments is crucial for our success in active combat. Often, our elite soldiers choose shotguns or pistols in CQB operations because of the impracticality of a long rifle. The capability of twisting corners and moving quickly is what makes our most elite soldiers so formidable.

The 5.5" barrel of the Maxim Defense PDX is for our nation’s most elite soldiers who need to move fast in tight spaces and have enough firepower to take on multiple insurgents at the same time. 

I would recommend that if you with a shorter barrel than 11", you make sure you get a silencer or muzzle brake attached to deal with the added percussion of the SBR. Something to always keep in mind is the shorter the barrel, the more noise you’ll hear and recoil that you’ll feel. 

Maxim Defense PDX

A lot of SBRs get it wrong by not generating enough energy for the heavy rifle-caliber bullet to generate enough speed to be effective. However, the biggest feature that Maxim Defense PDX has is the HATEBRAKE Muzzle Booster.

The HATEBRAKE optimizes the gases to ensure proper flow of as to expel the bullet at velocities above 1,800 FT/S.

Maxim Defense PDX

$2,295.00
at Maxim Defense
Prices accurate at time of writing

Very rarely will you ever find a rifle with a 5.5" barrel and an overall length of 18.75" that can fire heavier bullets accurately and consistently without malfunction.

The Maxim Defense PDX allows every operator the advantage of using an SBR with the power of a traditionally much larger rifle.

Specs

PDX Specifications 
Calibers 
  • 7.62x39mm
  • 5.56 NATO
  • .300 BLK
Barrel Length5.5"
Overall Length18.75"
Weight5.69 lbs
Barrel MaterialNitride coated
Twist Rate
  • 1:7=5.56mm
  • 1:10=7.62x39mm
  • 1:5= .300 Blackout
Muzzle AttachmentMaxim Defense HATEBRAKE Muzzle Booster
StockMaxim Defense SCW stock system
Barrel ProcessButton-Cut
TriggerALG Combat Trigger (ACT)
MSRP$2,295.99
 
What Ammo is the Barrel Tested/Optimized For?
 
As I’ve mentioned in many of our reloading blogs, the type of ammo you’re using matters so much more than many new shooters think.
Just like any other gun, they optimize the PDX with specific ammo that’s meant to give you the best combination of bullet velocity, force, and accuracy. 
 
Optimized Ammo For PDX5.56mm.300 BLK7.62×39
Bullet NameNATO M855 62gr ammunition(Fort Scott) supers 115gr and subs 190grFort Scott 117gr
 

MAXIM DEFENSE 7.62x39mm 117Gr TUI SCS

$37.90
at Gritr Sports
Prices accurate at time of writing

Why is the PDX the Best CQB Rifle?

If you’re looking for a rapidly deployable AR with the size of a large pistol, the PDX is probably for you. Not only is the SCW stock an amazing 4" (completely collapsed), it feels like it belongs on a $2,200 gun.

How many times have you used a stock on an SBR and it felt like cheap plastic that wouldn’t absorb any recoil at all? The SCW stock is sturdy and handles the recoil like no other SBR, I’ve ever fired. 

The maneuverability I have with the PDX almost makes me feel like I’m holding a big pistol with formidable power and unmatched ergonomics.

Many people talk about how a 5.5" barrel isn’t long enough, and I’d agree with that for certain situations like competitive shooting or combat at over 300 yards. However, the PDX is perfect for our military in urban warfare.

Maxim Defense PDX (5.56 NATO)

$2,629.99
at Guns.com
Prices accurate at time of writing

I can’t think of another AR that would be better than the PDX as a home defense gun, other than the MK18 from Daniel Defense. With an 11" barrel, it offers great versatility for a soldier.

Even with the added versatility of the MK 18, a 5.5" barrel that can accurately shoot targets at 200 yards away is nothing short of revolutionary in the gun industry. 


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