I am buildings my first SBR and am debating on the barrel length to go with. It’s going to be in 5.56. I have a SLR adj gas block in .750 ( will trade/sell for .625 if needed) I plan on running on this build. I would like to be able to reach out to 300m, light weight is important to me.
A DD MK18 10.3" barrel seems to be the obvious choice however I have noticed the popularity of 11.5/12" barrels. This has me interested in the advantages of the additional length. I realize a longer barrel equals greater velocity but am curious if there is a significant gain from a 10.3/5 to an 11.5/12 barrel.
Here’s a quote from the owner of BCM and why I went 11.5.
“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.
Thanks for mentioning this… Helpful quote. I thought about this and it is partly why I chose to run an adjustable gas block on this gun. My train of thought is that an adj gas block will enable me to increase the short dwell time inherent in the car length 10.5ish" barrels.
As per Paul’s advice on the 11.5" being the shortest, I would agree. Going below that is doable, but has considerable drawbacks for only an inch or so reduction in length. Dwell time, fierce muzzle blast, carrier speed, etc.
I have taken a liking to 12.5", but to each his own.
Dwell time is the time the bullet spends in the barrel between the gas port and the muzzle. It’s determined by bullet velocity and the length between those two points. An adjustable gas block adjusts the gas pressure in the barrel after the bullet passes the gas port, has nothing to do with dwell time. You might be able to get around a short dwell time by using an adjustable gas block, but in my mind, that’s useful primarily if you’re going to be switching between suppressed and unsuppressed configurations, or if the mfgr has overgassed the weapon by using too large of a gas port. An 11.5 gives you just that much more cushion (40% more, apparently) for more reliable rifle function. You do have to be selective about the barrel you choose. My 11.5 with BCM barrel functions perfectly with an H3 or H3 buffer. Outside of suppressor use, putting an adjustable gas block on has always seemed like a kludge to me.
I had a 7.5" that ran very well, but I had to use a more powerful buffer spring to get it that way. My 10 year old 9.5" (I used this upper in Iraq) runs perfect on a standard carbine spring and buffer. It, however, uses a “gas trap” system and that changes the pressure curve in the system. I’ve read a lot on SBRs over the years, and were I to start from scratch today, I’d go 11.5" and use the A5 buffer system. If size was the most important thing, and I had to run a can, I would go 10.5" and run a mini. Not sure ArmsTech (who I got my Compak 16 upper from) is even still in the game. I did have them install a rail on it maybe 2 years ago, but their web presence is almost non-existent. I had to call the guy on the phone, and they do business over Skype… PITA. I haven’t had any problems with the upper. I love it, but other companies seem easier to deal with.
I am seeing a lot of support for 11.5" here. I have read that .071 is the correct port size for 10.5" barrels, but what is the correct size for 11.5" barrels? How much does this change due to that extra 1" of barrel length?
As BCM noted there is an increased margin of reliability, the difference in length also allows XM193 to have a frag range out to around 40 yards vs. almost no frag range out of a 10.3. XM193 is not ideal, particularly out of an SBR but it’s nice to know it can be effective out of an 11.5… And the number one reason to go 11.5, KAC CQB upper is 11.5 and not 10.3/10.5 and IMHO on a suppressed SBR, the E3 bolt really shines.
I second that… But to save you the blisters on your fingers and trouble of writing what I am sure could be a very long post, would you mind telling me a little bit about your decision making process for the barrel you chose? I read the article you linked…helpful info. Thanks.
I originally wanted a 14.5" barrel. My local dealer talked me into a 10.5" bbl. I took into account that a 556 Spec war can will make the overall length just a bit longer than a carbine rifle (when can is attached). This was a primary factor for bbl length - outside of my local dealer’s opinion.
Gas system - carbine:
I started with a yhm lo pro gas block. The yhm did not fit well under my MI Mlok handguard. The gas block was making contact with the handguard. It caused the point of impact to shift 8" left (with the can on). I had cycling issues too.
I went to a bcm lo pro gas block. It is slimmer. It allowed for a better/ more accurate grouping - but - it was causing bounce back with the bolt. The bolt was not picking up rounds, it was cutting the cartridges wide open - powder spilling out.
Third round: jbo adjustable gas block. I tuned the rifle to function very well with the can on. It will not function with the can off. Im currently trying to figure out how to get it to cycle with & without can. The adjustable gas block is a real laerning curve. You barely open it up with the can on - you open it almost full gas w/o the can.
It shoots real nice & has very low recoil as it is now.
Oh & it has real nice groups at 100 yds. I can hit 300yds real well too.
I Have used 55gr up to 77gr bullets & they hit close to each other at 100 yds. I really like this 10.5" barrel. Its a Seekins 223 wylde in stainless - carbine gas.
Are you running a full mass BCG? What weight is your buffer? I am not an expert so take my advice with a grain of salt but It sounds to me like your over gassed. Changing GB and/or switching to a heavy buffer may get you right. Maybe try changing the buffer first as its a cheaper fix than dropping $120 on a high end adj gas block.
I researched gas blocks a lot for this build. Being a cheapskate I considered a budget ADJ GB but was talked out of it… Buy once cry once. SLR rifleworks sentry series came really highly recommend. You probably don’t want to buy yet another gas block but I would suggest giving SLR a call. Right To Bear has them for $105 which is a good deal.
My buddy has one on his 10.5 it runs well opened up 1/3 of the way un suppressed. He also runs a can on the gun but I am not sure which setting… He said its 2 or 3 /15.