Troubleshooting a new build, think I got it figured out.

[friendlyfireisnt]

Decided to put together a new build from some existing parts, and ran into a little trouble a the range. Think I got it figured out, but want to get others thoughts on this as well.

Details are:
11.3" Ballistic Advantage Barrel with pinned LPGB
Larue BCG from previous build
Aero Precision Upper
ALG V2 EMR
BCM receiver extension, spring and H2, H, carbine buffers.

Put the rifle together this week, no complications with the assembly. Brought it to the range for the first time on Friday. First range trip was to sight in the Aimpoint ML3 and check function. Prior to going to the range, the barrel and chamber were cleaned, and the BCG was lubricated generously with slip 2000 EWL.

Range Trip #1

Started off with Wolf Gold ammo and a Gen3 20rnd PMAG with the H2 buffer. Initial function showed poor ejection, with a few empties barely clearing the ejection port. After a few rounds, got my first failure to extract fully/eject. Fired case was out of the chamber, but still in the ejection port. New live round was attempting to feed below the fired empty case.

Changed out to an H buffer, ejection got a little better, but still had the same failure to eject. Changed to a carbine buffer, same result. Happened pretty consistently.

Changed out the BCG for a known good unit (Rainier Arms from a complete upper from them). No more failures to eject. Functioned 100% from that point on, both with Wolf Gold and with Federal Fusion (my Defensive loading).

Came home, pulled both BCG's. Found the Larue BCG's extractor tension was not very stiff. Had the older style spring, with a black insert, and no o-ring. Checked the Rainier BCG, which had the older style spring, and a brownish o-ring and black insert.

Carrier key doesn't appear loose, and is not showing any gas leaks. LPGB isn't showing any gas leakage either.

Pulled the Bolt from the Larue BCG, replaced it with a brand new BCM bolt. BCM bolt did not have the O-ring installed, but of course had the better spring and the black insert.

Ran several dummy cartridges through, repeatedly. No failures noted.


Range Trip #2

Took the rifle to the range again today, loaded up 104 rounds of Wolf Gold. Brought the Larue BCG with the new BCM bolt, brought the O-ring separately, and brought along a known good complete BCM BCG (black insert, no o-ring). Brought a couple Gen3 PMAG's, a Lancer mag and a USGI with Magpul follower. Kept the Carbine buffer, but also brought along a BCM "H" buffer.

With the Larue BCG/BCM bolt combo, on the 2nd round, had the same failure to eject. Fired case was just about clear of the chamber, but hadn't ejected, bolt was no longer holding on to the case. Next live round was attempting to feed, but didn't have a clear path to the chamber.

Cleared the malfunction, and tried again. 2nd round had the same thing occur.

Replaced the Larue BCG with a complete BCM BCG.
Fired the rest of the magazine with no failures.

Put the Larue BCG back in, had the same failure. again on the 2nd round.

Pulled apart the new BCM bolt on the Larue BCG, and put the o-ring over the extractor spring. Replaced that bolt/BCG back in the rifle.

Shot all remaining rounds with no failures of any kind. Fired cases are landing at the 4-O'clock position, about 10' from the rifle.



To break this down, here is what I have found:

BCG #1 with standard extractor spring, black insert, no 0-ring - Repeated failures to extract fully/eject.
BCG #2 with standard extractor spring, black insert, with o-ring - Zero Failures.
BCG #1 with new bolt with upgraded spring, black insert, no o-ring - Repeated failures to extract fully/eject.
BCG #1 with new bolt with upgraded spring, black insert, with o-ring - Zero Failures
BCG #3 with upgraded spring, black insert, No o-ring - Zero failures.

So for now, I've got the new BCM bolt with the o-ring in the rifle, with a carbine buffer. Planning on ordering an extractor upgrade kit for the the Larue Bolt, which I will then test when it comes in.

What confuses me is that the BCM BCG with no o-ring on the bolt had no failures; but a BCM bolt with no o-ring on the Larue BCG had repeated failures, until I put the O-ring in. Doesn't make sense to me, since they use the same extractor spring, insert, etc.

I think I have the failures solved, I'm just trying to wrap my head around why one BCM bolt requires the O-ring, while the other doesn't.

Any thoughts?

[gaijin]

I replace any BCG's extractor springs with BCM springs as a matter of course.
Saves a lot of headaches.

Too many instances of new guns, new builds with problematic extraction, weird double feeds, etc.
99% of the time replacing extractor spring with BCM relacement resolved issues (assuming the BCG wasn't complete crap to begin with).

[MistWolf]

Any extractor spring that needs an O ring to work needs to be replaced. Based on personal experience, I recommend the Colt M4 spring.

[friendlyfireisnt]

I replace any BCG's extractor springs with BCM springs as a matter of course.
Saves a lot of headaches.

Too many instances of new guns, new builds with problematic extraction, weird double feeds, etc.
99% of the time replacing extractor spring with BCM relacement resolved issues (assuming the BCG wasn't complete crap to begin with).

Agreed. Doesn't explain the failures with the brand new BCM bolt until I put in the O-ring though.

[friendlyfireisnt]

Got it figured out!

So when I came home from the range the first day, and I took apart the the various BCG's, I grabbed the suspect unit (Larue), the one that worked (Rainier) and a known good one (BCM).

I compared all three, as mentioned, and noticed the difference. Larue had the standard spring, black insert, no O-ring. Rainier had the standard spring, black insert, With O-Ring. The BCM had the HD Spring, black insert and no O-Ring.

So when I grabbed still in the wrapper spare BCM bolt I had laying around, I never looked to see what spring it had, in comparison to the other BCM. I didn't pull it apart in fact, until I had failures, and had to add the O-Ring.

So now I just pulled apart 2 complete BCM BCG's and the new BCM spare bolt. The complete BCG's had the upgraded HD spring, the spare bolt had a standard spring. Don't know if it's due to how long ago I purchased the bolt, or if they just don't upgrade the springs on the bolts by themselves.

Anyways, that would explain why switching bolts had no initial effect on the carbine's function, until I added an O-ring.

Looks like I will be ordering a pack of BCM extractor spring upgrades here shortly.

[tehpwnag3]

Anecdotal, but I've read several times that BCM's enhanced reliability kit has a stronger spring than the stock spring their complete BCG's come with.

[Jmedic_]

I have the BCM ion-bonded BCG. Recently, I bought the "sopmod" bolt upgrade kit to have, just in case. I read a few threads stating how the kit is basically useless and unnecessary. So I emailed BCM and asked if I install this kit in my BCG (that has 0 rounds on it) would it be an improvement, detrimental, or make no difference at all. They advised that installing the kit would be very beneficial, especially w/ a suppressed SBR. I installed the kit, and all the factory components of the BCG were the same as the upgrade kit, except for the spring. The spring that came w/ the BCG is much smaller than the spring in the upgrade kit. And, the spring definitely feels a lot stiffer. I haven't gotten a chance to get to the range, but I will report back my results when I do.

[tehpwnag3]

For someone who already has a BCM BCG, I agree with everything you've said.

This is a link to the kit you should have gotten: https://www.bravocompanyusa.com/BCM-...exspring-1.htm

I have the BCM ion-bonded BCG. Recently, I bought the "sopmod" bolt upgrade kit to have, just in case. I read a few threads stating how the kit is basically useless and unnecessary. So I emailed BCM and asked if I install this kit in my BCG (that has 0 rounds on it) would it be an improvement, detrimental, or make no difference at all. They advised that installing the kit would be very beneficial, especially w/ a suppressed SBR. I installed the kit, and all the factory components of the BCG were the same as the upgrade kit, except for the spring. The spring that came w/ the BCG is much smaller than the spring in the upgrade kit. And, the spring definitely feels a lot stiffer. I haven't gotten a chance to get to the range, but I will report back my results when I do.

[friendlyfireisnt]

For someone who already has a BCM BCG, I agree with everything you've said.

This is a link to the kit you should have gotten: https://www.bravocompanyusa.com/BCM-...exspring-1.htm

That's what I will be purchasing, probably atleast 2 of the 3-packs.

[titsonritz]

I can't believe people still dick with O-rings.