Best barrel length for 5.56 SBR

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  • turnerdye1

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    10.5" or 11.5". Most suppressor warranties wont cover anything shorter then 10.5" barrels. The 11.5" guns at work tend to work a little better then the 10.5", I believe it has something to do with the dwell time and cycling. That extra 1" gives you more back pressure to cycle the bolt better.
     

    aclark

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    Paul from BCM says 11.5", but admits there are plenty of 10.5" guns that run perfectly well. Here is his post from a well know AR-15 forum.

    Q: Why BCM chose the 11.5" SBR over the 10.5"

    I get this question a lot. It is a good one. When we spec our program we can build anything we would like, so we sat down and looked at the pros and cons.

    First Statement: I know a good 10.5" SBR can run very well. I personally own a semi MK18 type and it runs great. No problems.

    A: Dwell time.
    Dwell time is the time that your gas operated weapon maintains pressure to continue the cycling of the weapon. It primarily exsists from the time the bullet passes the gas port in the barrel to the time the bullet exits the muzzle. When you pull the trigger and fire the weapon the movement of the bolt carrier group unlocks the bolt, extracts, and ejects the spent casing. Then it cocks the weapon, feeds, chambers the next round, and then locks again. One of the thing that can make SBRs finicky is the dwell time (or lack of).

    The 11.5" carbine is approximately 4% longer weapon than its' 10.5" counterpart, but this extra inch gives the barrel a 40% increase in length for dwell time. IMHO, this is an excellent trade off. This additional dwell time (all other things being equal) will allow the carbine to be more forgiving to different ammo types, extremes in air temperature, weak or worn extractor springs, worn extractors, buffer weights, etc.

    Last Statement: For those folks who have a 10.5" that works very well, I would reply; me too. (see first statement)

    If I had to "go to war" with a AR15 Carbine, I would grab the BCM 11.5".
    The BCM 11.5" Runs like a dream.

    Hope this info helps,
    Paul
     

    KJQ6945

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    The 10.5" inch guns get kind of a bad rap on the Internet for being troublesome. I believe the problems arise from home built frankenguns, with incorrect parts.
    I've had zero issues with my 10.3" Colt. The Daniel Defense MK18 is also a very nice gun, without issues.
    If you are gonna build it yourself, do your homework and get the right parts the first time. Good luck.
     

    rvb

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    "best" means different things to different folks.
    shortest? most reliable? least wear on a can? lightest?

    have to figure your goals/purpose. for me, 12.5" was "best." my goal was perfect reliability and to loose as little velocity as possible. I just wanted to make the rifle+can roughly as long as a regular 16" gun and help the balance w/ the can attached.

    -rvb
     

    avboiler11

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    I have a 10.5" pistol waiting on a Form 1 to come back; it is overgassed a bit (throws brass at 1-1:30 even with a H2 buffer) but has never had a functional issue.

    I wouldn't want to get hit from a 7.5" 5.56, especially with expanding ammo suited to lower velocities (Mk318, Hornady GMX, Fed Fusion, etc)...but that's a LOT of blast and noise to deal with and pistol gas 5.56 weapons have historically been far more problematic than 10.5" carbine gas weapons
     

    daddyusmaximus

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    I had a 7.5" frankengun I pieced together and was very lucky with it. It ran very well with an extra power "red" spring. Kinda miss it. My favorite is my 9.5" SBR I built using an Arms Tech Compak-16 upper I used in Iraq on my M4 lower. Having used it as designed, it has a special place in my heart. Plus, almost nobody has a 9'5" gun. It runs perfectly, and it's gas trap system is somewhat rare. If I was building today I'd have no problem going with either 10.5 or 11.5. I would lean towards the 10.5 as the whole reason for a SBR is to get the small package. The DD MK18 would be a good choice for me.
     
    Last edited:

    throttletony

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    Besides reliability, you'll generally want the longest setup that you can tolerate. (in & out of vehicles a lot?, bedside? kicking doors?)
    LESS blast/concussion/flash. MORE velocity.
    If it's in .300BLK, I'd say 8.5-9.5" is great!
    For 5.56, I'd say 10.5-11.5" is great!
    Can you go shorter? Sure, but the trade off for 1-2 less inches (in my opinion) doesn't give enough benefits to overcome the lost velocity and increased flash/concussion.

    I'm not an expert on this.
     

    avboiler11

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    The big knock on a short 5.56 is velocity, ie. not enough to have a reasonable fragmentation range.

    With expanding ammunition, the velocity required for terminal effects is lower (sometimes significantly so), extending the same platform's effective range.
     

    NyleRN

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    I had a 7.5" frankengun I pieced together and was very lucky with it. It ran very well with an extra power "red" spring. Kinda miss it. My favorite is my 9'5" SBR I built using an Arms Tech Compak-16 upper I used in Iraq on my M4 lower. Having used it as designed, it has a special place in my heart. Plus, almost nobody has a 9'5" gun. It runs perfectly, and it's gas trap system is somewhat rare. If I was building today I'd have no problem going with either 10.5 or 11.5. I would lean towards the 10.5 as the whole reason for a SBR is to get the small package. The DD MK18 would be a good choice for me.
    That sir would be an LBR not an SBR :):
     

    wsenefeld

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    If you're going to suppress it, you don't have to worry about 10.5" being too short. The suppressor will increase back pressure which will overcome any decrease in dwell time.

    Since the velocity will significantly slower, you'll be best served with a heavier bullet. For home defense I use 75gr Hornady TAP (I wouldn't grab the AR first but if that was my only option...)
     

    Psode27

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    I have a 100% reliable 7.5" SBR. I shoot everything from cheap steel cased stuff, to various decent brass cased stuff. That being said there are those that say 100% reliable isnt "good enough". You must watch the rifle in slow mo video etc.... I am not one of those people though. I have no reason to not trust its reliability, and would be comfortable to take it anywhere with me. Sorry, I'm rambling. (This is my INGO therapy after a really long day)

    If I did it all over again I'd go 10.5. While 11.5 MIGHT be better, the more inches I add, the less "value" I see in my tax stamp... My 7.5 is fun, It gets lots of looks at the range, but when it comes down to it, I'd be better served with a 10.5". In your case, with suppression being a factor, 10.5 is generally good to go.
     

    Bfish

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    What did you end up with 09hemi? There were tons of good deals over the weekend! I have a 10.5" MK18 upper showing up tomorrow, and I'm hoping it runs well!
     

    rvb

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    The big knock on a short 5.56 is velocity, ie. not enough to have a reasonable fragmentation range.

    for me, 12.5" was "best." my goal was perfect reliability and to loose as little velocity as possible.
    -rvb

    I know this is an old thread, but I finally got around to collecting some chrono data.
    55gr Hornady FMJBT w/ 25.0gr H335 ... my go-to plinker round. 2879 out of my 18" 3-gun rifle (woa barrel w/ wylde chamber). 2677 out of my 12.5" SBR (BCM barrel w/ 5.56 chamber).

    I thought loosing only 200fps was pretty solid. I'd be curious to see this same load in 11.5", 10.5", etc....
    I'll have to take a 20" and 16" next time...

    but one of the reasons I went 12.5" was to reduce velocity loss, and I'm pretty happy w/ the results. Lost a few fps with the can attached (2640 iirc).

    -rvb
     

    romack991

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    I thought loosing only 200fps was pretty solid. I'd be curious to see this same load in 11.5", 10.5", etc....

    Similar load, 55gr Hornady SP w/ 24.5gr H335 @ 2.200 oal.
    18" Seekins (223 wylde) - 2887 fps
    16" Stretch16 (223 wylde) - 2827 fps
    11.5" Faxxon Big Gunner (5.56 NATO) - 2580 fps
     
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