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
That sir would be an LBR not an SBRI 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
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 thought loosing only 200fps was pretty solid. I'd be curious to see this same load in 11.5", 10.5", etc....