Is there an optimal barrel length for an AR-15 SBR?

[fred333]

I just submitted a Form 1 for a [stripped lower] AR-15 build (.223/5.56) with a 12.5" barrel (OAL 29.1") intended to be used both with and without an Optimus silencer. I'm wondering if I could get away with a barrel that's an inch or two shorter without seriously affecting both reliability or muzzle velocity? Anywhere around ~10.5"~12.5" is fine. I just wanna see if there's a sweet spot in there that I should be aiming for?

I realize it's difficult, if not impossible, to gage ideals without identifying a specific purpose or task for a given build. Still, I wanted to see what the pros and cons are?

 

[DutchV]

10.5" seems to be the short end of consistently reliable 5.56 barrel length, without losing too much velocity. Any shorter heads into fireball territory.

 

[vgplayer]

I know BCM recommends and doesn't make any shorter than 11.5" for 5.56 because of the increased dwell time and reliability. For short range stuff I think there is still plenty of velocity even with 10.3".

You must have a really short stock if 12.5" barrel gets you only to 29.1" OAL. I think the consensus here is a 10.3" barrel can get you 29.1" with a magpul stock.

 

[fred333]

I know BCM recommends and doesn't make any shorter than 11.5" for 5.56 because of the increased dwell time and reliability.

I've read that about BCM's preference for 11.5" and I'd originally submitted the Form 1 spec'ing that barrel length. But, after reading a number of articles suggesting the additional inch would be better (i.e., the gun'd be more reliable, more accurate and easier on the silencer), I deleted the original submission and re-submitted with a 12.5" barrel length. And that's sorta the background for my question. Do the benefits of the additional inch—going from an 11.5" barrel to a 12.5" barrel—result in tangible benefits (over something shorter) or is it a case of diminishing returns unless I add still more length (e.g., 14", 16"+)? Are there any downsides re velocity/reliability at 12.5" that would be mitigated by a shorter 10.5" or 11.5" barrel?

You must have a really short stock if 12.5" barrel gets you only to 29.1" OAL. I think the consensus here is a 10.3" barrel can get you 29.1" with a magpul stock.

Since I can't legally assemble the gun to verify the exact length, I'm estimating the average AR-style stock'll get me ~29.1" with a 12.5" barrel. If that's incorrect, maybe I should delete the current submission and re-submit again?

 

[OrbitalEllipses]

Since I can't legally assemble the gun to verify the exact length, I'm estimating the average AR-style stock'll get me ~29.1" with a 12.5" barrel. If that's incorrect, maybe I should delete the current submission and re-submit again?

You're fine. My 11.5" is in the 31" area with an A5. With a standard carbine extension it'd be in the 30" area.

 

[mopar92]

11.5" is simply the most tested reliable short barrel length dating back to the XM-177 and later the GAU-5/Commando and todays Mk.18 (although the Mk. 18 may be 10.5 can't remember offhand).

With any short barrel you get decreased reliability as there is less dwell time (the time that the projectile is still inside the barrel) and increased recoil due to lighter weight and increased chamber pressures as most manufacturers will have increased the size of the gas port to maximize scavenging and the action will be more violent as the casing will be extracted under higher pressure forces.

 

[teratos]

I've got a few thousand reliable rounds through my 10.3" 5.56. No hiccups at all. Both suppressed and unsuppressed with good and crappy ammo. It's also > 29" with a Magpul stock.

 

[E.Shell]

I have a 10.3" pistol with about 300 rounds through it and it has run fine thus far with 55 grain ammo, both M-193 and commercial. We'll see how it acts when it gets caked up enough to need cleaning...

 

[rouchna]

I've got a few thousand reliable rounds through my 10.3" 5.56. No hiccups at all. Both suppressed and unsuppressed with good and crappy ammo. It's also > 29" with a Magpul stock.

Same here. My DD MK18 (suppressed and unsuppressed) upper has worked flawlessly on my SBR.

 

[clandestine]

All things being equal, in my experience shorter barrels can more accurate.

Shorter can = Stiffer.

NOT all Suppressors can suppress a 10.3, so check with the Manufacturer and MAKE SURE your barrel stabilizes projectiles before installing a Can.

10.3 is the MK18 nd can be made reliable, BUT there are things that I believe will make the gun more reliable.

Use a Hard Chromed BCG or a Phosphate/Chrome Lined BCG (No NiB or Nitride)
Use a Colt Gold Extractor Spring (DONT use an O Ring with this spring, its NOT needed)
Use a Colt Action Spring or a Tubb Flat Wire Action Spring
Use Chrome Lined Barrel (If you like Nitride, it works better than unlined/untreated)

If it will be shot suppressed and unsuppressed, a CLICK Adjustable or Adjustable (Auto Bleedoff) Gas Block are optimal.

Dont use a Notched Hammer
Dont use an Unshrouded Bolt Carrier
Dont use 1 Piece Gas Rings
Dont use KNS Firing Pin Retaining Pins

 

[Sticky]

Ballistically, with a 223 or 556 cartridge, you do benefit from every inch of bbl length, but in an SBR or pistol configuration, I think as long as you are at 10.5" or thereabouts, you are fine. It will run, function fine and not be too obnoxious (unless you stick a brake on it). The length of the gas system determines pressures along with the gas path sizes, an inch or two really doesn't make that much difference, unless you get to the point that you can go to a longer gas system.

 

[outrider58]

All things being equal, in my experience shorter barrels can more accurate.

Shorter can = Stiffer.

NOT all Suppressors can suppress a 10.3, so check with the Manufacturer and MAKE SURE your barrel stabilizes projectiles before installing a Can.

10.3 is the MK18 nd can be made reliable, BUT there are things that I believe will make the gun more reliable.

Use a Hard Chromed BCG or a Phosphate/Chrome Lined BCG (No NiB or Nitride)
Use a Colt Gold Extractor Spring (DONT use an O Ring with this spring, its NOT needed)
Use a Colt Action Spring or a Tubb Flat Wire Action Spring
Use Chrome Lined Barrel (If you like Nitride, it works better than unlined/untreated)

If it will be shot suppressed and unsuppressed, a CLICK Adjustable or Adjustable (Auto Bleedoff) Gas Block are optimal.

Dont use a Notched Hammer
Dont use an Unshrouded Bolt Carrier
Dont use 1 Piece Gas Rings
Dont use KNS Firing Pin Retaining Pins

I've been saying that for years!

But seriously folks, I've followed these words on my 10.5 build and 400-500 rounds later shooting just about every kind of 5.56, .223, reloads suppressed and not suppress and Zero problems to date. I do not shoot steel cased ammo.

 

[clandestine]

I've been saying that for years!

But seriously folks, I've followed these words on my 10.5 build and 400-500 rounds later shooting just about every kind of 5.56, .223, reloads suppressed and not suppress and Zero problems to date. I do not shoot steel cased ammo.

I knew that advice would not go unchallenged.