What is BCM's "Mil-Spec Crane O-Ring" for?

[Solidsnake87]

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.

[BAGunner]

HD extractor springs & Crane O-rings

[damon1272]

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.

[Solidsnake87]

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.

[tomd1584]

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.

[sleepercar]

Better to need and not have, then to need it and not have it.

Something like that...

[Ruiner]

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.

[tomd1584]

Something like that...

haha, just got home and realized what i was typing while driving.

[erratikmind]

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.

[God Bless The Mauser]

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.

[sleepercar]

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..

[ar15barrels]

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.

[TKM]

It's a basic Keltec SU16 upgrade as well.

http://www.ktog.org/cgi-bin/yabb2/YaBB.pl?num=1141021271

[ar15barrels]

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.

[RT13]

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.