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Centerfire Rifles - Semiautomatic or Gas Operated Centerfire rifles, carbines and other gas operated rifles.

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  #1  
Old 08-25-2010, 5:29 PM
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Default What is BCM's "Mil-Spec Crane O-Ring" for?

One of these O-rings came with a bolt group. What is its purpose? I've never seen one on any of my other bolt groups.
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Old 08-25-2010, 5:36 PM
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HD extractor springs & Crane O-rings
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Old 08-25-2010, 5:38 PM
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Those are to be used with the CAR or M4 gas system to provide more tension on the extractor for the harder extraction that takes place with teh short gas system. If you have a mid-length or rifle length gas system then it is not needed.
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Old 08-25-2010, 5:47 PM
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I'm hoping the one in the package was a spare then, because I can't find it. Then again, it would seem strange to force a customer to disassemble a bolt to install an O-ring if it could have been pre-installed to begin with.
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Old 08-25-2010, 6:05 PM
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Quote:
Originally Posted by Solidsnake87 View Post
I'm hoping the one in the package was a spare then, because I can't find it. Then again, it would seem strange to force a customer to disassemble a bolt to install an O-ring if it could have been pre-installed to begin with.
I believe its an extra, in addition to one already installed; in any case, they're not forcing you to do anything. If you need extra tension, install it. If you dont, dont. Better to have and not need, then to need it and not have it.

Last edited by tomd1584; 08-25-2010 at 6:47 PM.. Reason: haha, dont text and drive!
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Old 08-25-2010, 6:13 PM
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Originally Posted by tomd1584 View Post
Better to need and not have, then to need it and not have it.
Something like that...
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Old 08-25-2010, 6:15 PM
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It isn't installed. They include it if the customer wants to install it. You don't need it with a BCM bolt as it comes with a better extractor spring already.
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Old 08-25-2010, 6:47 PM
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Something like that...
haha, just got home and realized what i was typing while driving.
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Old 08-25-2010, 7:31 PM
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I recently ordered a 16" middy BCM BFH upper with the BCM BCG. The bolt assembly and the O ring were sealed separately in a plastic bag. I took the extractor off to see if a O ring was already installed around the spring. It was not. So, I installed it.

Prior to taking my new rig to the range, I decided to check with Bravo Company and asked about the O ring. I was told "You probably will not need it as the BCM extractor springs are all ready stronger than Milspec."

Needless to say, I removed the O ring. I have since shot some 600 rounds in the past 2 weekends without any issues.
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Old 08-25-2010, 7:32 PM
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Mine put enough tension on it to rip the rim off the case while shooting Tula ammo. It was polymer coated and I hadn't cleaned the chamber for a few hundred rounds and it got stuck. This is the first major jam I've ever had with an AR but it was purely ammo related. It was actually my buddy shooting it when it happened, he also managed to get the AK to stovepipe and get the trigger jammed to the rear, something I couldn't replicate, maybe he should stick to bows.
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Old 08-25-2010, 8:52 PM
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haha, just got home and realized what i was typing while driving.
D'oh! While driving? Hell, I could barely text the wife "Yes" or "No" while driving. I usually just end up chucking my phone at the floorboard.
Any man that can text a decent response while driving is definitely allowed an error.. or 20..
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Old 08-25-2010, 9:45 PM
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Quote:
Originally Posted by Solidsnake87 View Post
I'm hoping the one in the package was a spare then, because I can't find it. Then again, it would seem strange to force a customer to disassemble a bolt to install an O-ring if it could have been pre-installed to begin with.
The o-ring is only desireable on carbine gassed barrels.
It's use on mid-length or rifle length barrels could cause failures to eject due to the extra extractor tension.
Since BCM has no way of knowing what barrel a bolt is going into, they leave it off, but include it so you can use it if you need to.
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  #13  
Old 08-25-2010, 10:03 PM
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It's a basic Keltec SU16 upgrade as well.


http://www.ktog.org/cgi-bin/yabb2/Ya...num=1141021271
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  #14  
Old 08-25-2010, 10:47 PM
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It's a basic Keltec SU16 upgrade as well.


http://www.ktog.org/cgi-bin/yabb2/Ya...num=1141021271
That's a good point.
Piston operated guns unlock quicker like a carbine gassed AR does.
The faster unlocking causes a reduction in extractor force via centrifugal force as the front of the extractor is heavier than the rear of the extractor.

Therefore, piston operated guns may see an improvement in extraction reliability with the o-ring, especially if the bolt is trying to extract before the bullet leaves the barrel.
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  #15  
Old 08-26-2010, 12:16 AM
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Quote:
Originally Posted by erratikmind View Post
I recently ordered a 16" middy BCM BFH upper with the BCM BCG. The bolt assembly and the O ring were sealed separately in a plastic bag. I took the extractor off to see if a O ring was already installed around the spring. It was not. So, I installed it.

Prior to taking my new rig to the range, I decided to check with Bravo Company and asked about the O ring. I was told "You probably will not need it as the BCM extractor springs are all ready stronger than Milspec."

Needless to say, I removed the O ring. I have since shot some 600 rounds in the past 2 weekends without any issues.
I ordered the exact same upper as yours, BFH 16"Middy. I have a question regarding the finish on the inside of the upper receiver (not the barrel). The outside finish is a nice matte black finsih on the receiver, but on the inside it is not. It's more of a grayish color and not as smooth. Is your upper the same? Thanks in advance.
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Old 08-26-2010, 2:51 AM
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Quote:
Originally Posted by RT13 View Post
I ordered the exact same upper as yours, BFH 16"Middy. I have a question regarding the finish on the inside of the upper receiver (not the barrel). The outside finish is a nice matte black finsih on the receiver, but on the inside it is not. It's more of a grayish color and not as smooth. Is your upper the same? Thanks in advance.
See post #3

http://www.m4carbine.net/showthread.php?t=55930

Is that how yours looks?
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  #17  
Old 08-26-2010, 4:27 AM
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Quote:
Originally Posted by RT13 View Post
I ordered the exact same upper as yours, BFH 16"Middy. I have a question regarding the finish on the inside of the upper receiver (not the barrel). The outside finish is a nice matte black finsih on the receiver, but on the inside it is not. It's more of a grayish color and not as smooth. Is your upper the same? Thanks in advance.
Yes, the interior of my upper is as you described. I noticed it when I initially was preparing the upper for the BCG. The exterior fit and finish is next to perfect and it matches my new Spikes lower very well. Having noted that, I contacted Bravo Company for the second time. I was told that The lighter appearance/unfinished look was due to a coating process, which required baking or something to that affect.

Needless to say, my new rig has performed flawlessly in the past 2 weeks after shooting some 600 rounds of PMC 55 gr. My best 50 yard 5 shot(apologies to you 10 shot folks) groupings have been between 1/2" to 3/4". The largest groupings have been consistently under 1 - 1/2". Another 50 yard session and I will move out to 100 yards, once I get the Troy sights and the EOTech XPS2-0 all dialed in. I will also be testing to see how my rig likes Hornady Tap and Silver Bear ammo, as well.

If you are interested in the ammo test results, I would be glad to PM you the results, so as not to further detract from OP's thread.
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  #18  
Old 08-26-2010, 7:29 AM
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Should we use the O-ring on an FN bolt that is used in an LMT 10.5 upper?
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  #19  
Old 08-26-2010, 7:56 AM
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Quote:
Originally Posted by RT13 View Post
I have a question regarding the finish on the inside of the upper receiver (not the barrel). The outside finish is a nice matte black finsih on the receiver, but on the inside it is not. It's more of a grayish color and not as smooth.
That's the dry lube that is sprayed inside the receiver.
Do NOT remove the dry lube.
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  #20  
Old 08-26-2010, 7:57 AM
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Quote:
Originally Posted by gunsmithcats View Post
Should we use the O-ring on an FN bolt that is used in an LMT 10.5 upper?
Only if it needs it.
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  #21  
Old 08-27-2010, 12:16 AM
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Thanks for the quick replies. The upper is great but compared to my Daniel Defense, when I noticed the inside of the BCM, I wasn't too sure if it was normal so just checking. Thanks.
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