Q: Why BCM chose the 11.5" SBR over the 10.5" (Page 1 of 2)
Posted: 5/13/2009 9:33:34 AM EDT
[Last Edit: BravoCompanyUSA][Edited]
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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 
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Thanks Paul - exactly the info I was looking for. Didn't realize 1" would be that much of an exponential difference  But happy now that I went ahead and ordered one of the BCM 11.5's a few days ago I have an H-buffer on my current SBR anyway, but wanted to get a quality upper/barrel for it. Now I just have to wait for Monday to roll around 
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Originally Posted By ziarifleman:
Is there enough clear barrel on these to mount an AAC SPR/M4? How much clearance is needed ? |
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Originally Posted By BravoCompanyUSA:
Originally Posted By ziarifleman:
Is there enough clear barrel on these to mount an AAC SPR/M4? How much clearance is needed ? 2.49" |
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2.49" measured from what point to what point ?
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Originally Posted By BravoCompanyUSA:
2.49" measured from what point to what point ? From the muzzle back. If there is anything that it gets into within 2.49" of the muzzle, then it won't work. I think the bayonet lug might be in the way, more than anything. |
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Originally Posted By ziarifleman:
Originally Posted By BravoCompanyUSA:
2.49" measured from what point to what point ? From the muzzle back. If there is anything that it gets into within 2.49" of the muzzle, then it won't work. I think the bayonet lug might be in the way, more than anything. 1-7/8" from bayo lug to end of threaded muzzle (with out flash hider installed). |
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Originally Posted By BravoCompanyUSA:
Originally Posted By ziarifleman:
Originally Posted By BravoCompanyUSA:
2.49" measured from what point to what point ? From the muzzle back. If there is anything that it gets into within 2.49" of the muzzle, then it won't work. I think the bayonet lug might be in the way, more than anything. 1-7/8" from bayo lug to end of threaded muzzle (with out flash hider installed). Thank you, sir. |
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| Well I guess its time to spend more money. I have a LMT 10.5 that I bought from Bravo that rides beside me on patrol. Now i gotta buy a BCM lower and then do the wait period for the NFA stamp to match it to the 11.5 BCM upper. This black rifle thing is really addictive. I wish all mine were BCM's. |
| Are the muzzle threads concentric with the bore enough to use these BCM 11.5 bbls with suppressors? |
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Originally Posted By eric10mm:
Are the muzzle threads concentric with the bore enough to use these BCM 11.5 bbls with suppressors? |
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They will be within the GI print spec.
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Originally Posted By eric10mm:
Would you put your $1000 suppressor on one? Sorry Eric, I did not see this reply. Yes. I have more times, on more barrels, with more supressors, than I can honest count. |
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Originally Posted By kennedyusmc:
Nice post... So what about the 11.5 middy? Longer gas system for more gas expansion (less pressure and smoother operation), but less dwell Which do you reccomend? Carbine or Middy 11.5? Middy gas on a 11.5' would not be combat reliable. It also would not fit with a FSB. |
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Originally Posted By BravoCompanyUSA:
Middy gas on a 11.5' would not be combat reliable. It also would not fit with a FSB. How about rifle gas system? 
This should be tacked with the SBR FAQ. |
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Originally Posted By Mopseydocks6014:
What advantages would the 12.5 have over the 11.5 if any?? More dwell time (not sure how significant it is in regard to reliability), ability to mount certain cans that wont fit on the 11.5", more velocity to shoot a little further, and, of course....more weight!
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Originally Posted By BravoCompanyUSA:
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 Math was never my strong suit, but 10.5 x 104% is only 10.92" It seems that 109.5% x 10.5 comes out to 11.4975" Am I mistaken and if so, where did I go wrong? ETA: Of course, assuming my math is correct - I now have to ask where the 40% more reliable number came from or how it was calculated. |
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Originally Posted By Maryland_Shooter:
Math was never my strong suit, but 10.5 x 104% is only 10.92" It seems that 109.5% x 10.5 comes out to 11.4975" Am I mistaken and if so, where did I go wrong? ETA: Of course, assuming my math is correct - I now have to ask where the 40% more reliable number came from or how it was calculated. The reference was to the carbine carbine, not barrel. 1" of a 25-ish inch gun is about 4% depending on the type of stock and where you like it to sit. GO BCM 11.5" !!!!
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Originally Posted By UVvis:
Originally Posted By Maryland_Shooter:
Math was never my strong suit, but 10.5 x 104% is only 10.92" It seems that 109.5% x 10.5 comes out to 11.4975" Am I mistaken and if so, where did I go wrong? ETA: Of course, assuming my math is correct - I now have to ask where the 40% more reliable number came from or how it was calculated. The reference was to the carbine carbine, not barrel. 1" of a 25-ish inch gun is about 4% depending on the type of stock and where you like it to sit. GO BCM 11.5" !!!!
http://i137.photobucket.com/albums/q223/db556/BCM61612.jpg Lost me doc. Carbine carbine? How does repeating a word modify it's meaning. Not being an ass, just trying to comprehend |
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| Paul's estimation is pretty close - you're only adding an inch or so to the OAL of the weapon system, but that inch is located in the most advantageous place in regards to gas system reliability. I've gone with 11.5" and won't look back. If you 'need' a 10.5", then just get any BCM carbine, and have it shortened. I'm happier with the 11.5" anyway. |
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Originally Posted By UVvis:
Originally Posted By Maryland_Shooter:
Lost me doc. Carbine carbine? How does repeating a word modify it's meaning. Not being an ass, just trying to comprehend Just a typo typo. 
I still don't see the math that shows 4%
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Originally Posted By Croft32: Originally Posted By Mopseydocks6014: What advantages would the 12.5 have over the 11.5 if any?? More dwell time (not sure how significant it is in regard to reliability), ability to mount certain cans that wont fit on the 11.5", more velocity to shoot a little further, and, of course....more weight! Not sure if 12.5 will give much benefit over 11.5 in regards to dwell time. There's no marking on the graph, or dedicated line for 12.5" in the table, but you can kinda rough your own  |
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Originally Posted By Maryland_Shooter:
I still don't see the math that shows 4% My 11.5" barreled carbines are about 27.5" long with the CTR stock collapsed and using an A2 birdcage hider. One inch of 27.5 inches is represents 3.6% of the weapon's overall length. A 10.5" barreled carbine, set up the same, would come in at about 26.5". Making one inch of the 10.5" carbine represent 3.8% of the carbines overall length. So realistically, one inch is adding less than 4% of length to the weapon system, for more dwell time. |
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Originally Posted By UVvis:
Originally Posted By Maryland_Shooter:
I still don't see the math that shows 4% My 11.5" barreled carbines are about 27.5" long with the CTR stock collapsed and using an A2 birdcage hider. One inch of 27.5 inches is represents 3.6% of the weapon's overall length. A 10.5" barreled carbine, set up the same, would come in at about 26.5". Making one inch of the 10.5" carbine represent 3.8% of the carbines overall length. So realistically, one inch is adding less than 4% of length to the weapon system, for more dwell time. My math: 10.5 x 104% is only 10.92" It seems that 109.5% x 10.5 comes out to 11.4975" Can someone show me their math showing 4%? Seriously, I'm still perplexed. I don't doubt what the claim is, but seeing the math would really help me understand it. |
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ME TOO I love your products and prices and can see your point when firing burst and full auto. That said, I have put thousands of rounds through my LMT 10.5 and have NEVER had a failure. This is of course, semi auto only. I do not frequently clean or oil my rifle on purpose because I use it on duty and I need to know that it will work every time. |
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Originally Posted By kap_x:
Originally Posted By Croft32:
Originally Posted By Mopseydocks6014:
What advantages would the 12.5 have over the 11.5 if any?? More dwell time (not sure how significant it is in regard to reliability), ability to mount certain cans that wont fit on the 11.5", more velocity to shoot a little further, and, of course....more weight! Not sure if 12.5 will give much benefit over 11.5 in regards to dwell time. There's no marking on the graph, or dedicated line for 12.5" in the table, but you can kinda rough your own http://ar15barrels.com/tech/pressure-time.gif 12.5" would give about .148 net time. Nice compared to a mid length 14.5" at .139 net time. |
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Originally Posted By 9divdoc:
Originally Posted By testudo:
That is a beauty! Wish I got one of these instead of my LMT 10.5" from Bravo Co. Yes you do...:) They are nice...glad I sold my LMT ..ya never know when we will see these again after this batch has flown the nest... I have both and could not be happier. I bought the LMT first and learned BCM was selling factory SBR's and bought one. I have the bug and think a 12.5" Kino from BCM is in my future. |
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I just have to have a KINO. But I can't wait any longer. So today, I ordered an 11.5.
I still need a Kino. But today, I ordered an 11.5. It'll go on a BCM lower that I SBR'd. So today, I orderd an 11.5. Is it here yet?
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The upper came today.
BCM 11.5" Upper BCM BCG BCM Gunfighter 4 charging handle Daniel Defense fixed rear sight Magpul MOE handguards. Mount-N-Slot Scout LIght mount Surefire Scout Light Mount-N-Slot Handstop Its not done yet. |
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Originally Posted By Lancelot: The new SBR with stock options. With CTR http://farm6.static.flickr.com/5255/5500215370_bf84bc4704_z.jpg  With EMOD http://farm6.static.flickr.com/5259/5500215922_9fbf290d8f_z.jpg  .  |
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