short bbl confusion

[m4forme]

i want to put togeather a SBR AR should i go with a 10 or a 11.5 in bbl... did a Bushmaster 11.5 years ago and could never get it to run right... that was before H buffers and the extactor upgrades... whose upper should i use BCM or LMT???
Rainer Arms has a LMT 10.5 in for $485... BCM's run about the same... What would you guys do????

Thanks

[RojasTKD]

Here is what Bravo Company said about why they choose to go with 11.5" uppers:




"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 "


I went with a BCM 11.5" for my SBR.

[Omega_556]

I choose an LMT 10.5 and love it, but I don't think you could go wrong with a BCM...

[spamsammich]

My BCM 11.5" upper ran like a top as an AR pistol. I wish I could have a proper SBR, I'd rock one right f'n now if it was legal in this stupid state..

[captshiess]

I have a 10.3 and 10.5" upper and both run awesome. I mostly went with the shorter one because my life doesn't depend on it. If it did, I would also probably go with the 11.5 for the extra velocity.

[m4forme]

think i'll go with the BCM 11.5 the extra inch past the gas port is a good thing...

[Artos]

Follow up Question:


If you were going to build 2 sbr uppers, and one was going to be your primary 11.5", is there any consideration or preference to go to a longer 12.5" over the 10.5"??

What is the trade off of the extra length velocity / dwell time vs. the compact 10.5?? I guess I should say these would be suppressed guns as well.

Any thoughts???