UPDATE 7/2 - Can a Carbine Require a "Warm Up" to Function Reliably?

[arptsprt]

Out to the range again today. I did what you suggested below. BCM bolt in LMT carrier, LMT bolt in BCM carrier.

The gun had extraction malfunctions with the BCM bolt/LMT carrier combo but not the LMT bolt/BCM carrier. Several FTE with the one round in the mag trick and semi-rapid fire. So, obviously the BCM bolt is bad... I am not going to screw with the BCM BCG anymore and will wait to hear from BCM on a solution this week. I am just going to keep the LMT BCG in the upper for the long run. It's been reliable for over 200 rounds.

IG; couple of observations/questions just to help learn for the future:

Noticed the ejection pattern was different with the bolt/BCG combos. The LMT bolt/BCM carrier combo rejected about 4 o'clock and 6-8 feet. The BCM bolt/LMT carrier combo was more 2-3 o'clock and about 4 feet. Does that tell you anything?

Also, at what point does one need to be concerned about switching bolts/BCG between rifles and headspace issues?

Thanks for the help.

I'm a little lost myself. This doesn't make sense. Take the bolt (only) from the LMT and put it into the BCM carrier. Then do the same with the BCM bolt into the LMT.

[Iraqgunz]

It could be simply variations in the ejector spring (and most likely is). As for the headspace issue, I wouldn't be concerned unless you have A LOT of rounds through the combination or barrels/bolts.

Out to the range again today. I did what you suggested below. BCM bolt in LMT carrier, LMT bolt in BCM carrier.

The gun had extraction malfunctions with the BCM bolt/LMT carrier combo but not the LMT bolt/BCM carrier. Several FTE with the one round in the mag trick and semi-rapid fire. So, obviously the BCM bolt is bad... I am not going to screw with the BCM BCG anymore and will wait to hear from BCM on a solution this week. I am just going to keep the LMT BCG in the upper for the long run. It's been reliable for over 200 rounds.

IG; couple of observations/questions just to help learn for the future:

Noticed the ejection pattern was different with the bolt/BCG combos. The LMT bolt/BCM carrier combo rejected about 4 o'clock and 6-8 feet. The BCM bolt/LMT carrier combo was more 2-3 o'clock and about 4 feet. Does that tell you anything?

Also, at what point does one need to be concerned about switching bolts/BCG between rifles and headspace issues?

Thanks for the help.

[six8]

I've had the exact problem with my SBR running a BCM BCG.. Haven't had the chance to try another bolt but I bet it's the problem

[arptsprt]

Interesting. What length is your SBR?

I have another factory complete BCM 11.5 BFH upper/BCM BCG with the DDMK18 rail that has been outstanding from day one.

I've had the exact problem with my SBR running a BCM BCG.. Haven't had the chance to try another bolt but I bet it's the problem

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[azoutdoorsman]

First, I have already contacted BCM again and I am sure I will hear from them next week...

Got the supposed repaired/replaced BCG back from BCM last week. I could tell it was the same one I sent back. Apparently their repair was insufficient. Had the same malfunctions multiple times again today when I went to the range to test fire.

Again today I once again tried an LMT BCG and another BCM BCG from another BCM rifle and both functioned in the upper just fine for several (over 100) rounds.

What doesn't make sense however, is I tried the problem BCG in my 16" BCM BHF rifle and it ran fine, albeit for only a mag's worth of rounds.

As a reminder, the upper with the problem BCG is a BCM 12.5 KINO about 450 rounds through it at this point.

For those who know better, why would this BCG work with the 16" and not the 12.5?

Longer dwell time in the 16 inch, especially if it's a carbine gas system.

[six8]

Interesting. What length is your SBR?

I have another factory complete BCM 11.5 BFH upper/BCM BCG with the DDMK18 rail that has been outstanding from day one.






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BCM 11.5" bbl and BCM BCG.

[John_Burns]

For those who know better, why would this BCG work with the 16" and not the 12.5?

The 16 inch is a mid gas and the 12.5 is carbine gas. You most likely have a weak extractor spring or faulty extractor.

The carbine gas system is putting a bit more stress on the extractor.

[arptsprt]

Thought I'd provide an update. I have still been battling malfunctions with this set-up. They evolved to a point where no ammo was cycling. One, maybe two round down the pipe then the extraction/feed malfunctions.

Through some much appreciated help from others on this forum, in particular IG, we were able to isolate the issue to a bad bolt. I have been in contact and working with BCM the entire time and they have really stood by me in working thru this frustrating issue. They really have not seen anything like this before and they are equally as frustrated.

Bottomline, I should have the upper finally up and running tomorrow. And while this has been a long process, and very frustrating, I am very pleased and appreciative of BCM's customer service and helpfulness. For what it is worth, I recently purchased a new BCM 11.5 BFH upper and it has been fantastic... This was just one of those unusual issues...

[TMS951]

Do you mind explaining in greater detail what exactly was wrong with the bolt.

Was it the actual bolt, or was it the one or some of its small parts (ejector, extractor, springs)?

[arptsprt]

Good question... Answer? We don't really know... That is what made this very, very frustrating for both sides.

A brief summary of the process with BCM:

They had me send back the original BCG. They said they evaluated the tolerances of the bolt itself and it passed. However, initial test fire in full auto resulted in failures to extract. So, they replaced the extractor and rings of the original bolt. Test fired again in both semi and full auto and it functioned fine. I got the repaired BCG back and I still had the malfunctions. The malfunctions now began with other ammo (original issue was with PMC Bronze).

Next, BCM had me send both the BCG and entire upper back for T&E. This time they replaced JUST the carrier. Verified the function of the URG. Test fired without issue at BCM. Got the URG back and malfunctions were even worse. BCM suspected the Spike's ST-T2 buffer and bad ammo were culprits. After discussion with BCM, I did replace the ST-T2 with a milspec H2 buffer. No go. Every ammo type I tried (GOOD .223/5.56) - no go. Got to a point where only one shot could be fired then the malfunction would happen. Tried different buffers, my other SBR lower. No Go.

I was able to isolate the bad bolt by doing a couple things. I swapped the original bolt in the carrier with a spare BCM bolt assembly and it ran. I tried an LMT bolt in the BCM carrier and it ran. Conversely, the bad bolt in the LMT carrier, no go. The last thing I did was once again replace the extractor, spring, and insert in the bad bolt with a spare BCM SOPMOD bolt rebuild kit (no o-ring) and oddly enough it ran.

Finally, out of frustration to prove I wasn't crazy, I took a few videos of all my issues while at the range for documentation and sent them to BCM. At that point, they sent me a complete new BCG. I ran the upper and new BCG very hard this past weekend and it ran great. All ammo.

The bolt cleared all tests they performed at BCM more than once but I could not get the thing to run. They do not understand why either. IG seems to think that, while rare, is there a possibility the bolt face depth, extractor position, or extractor spring pocket were out of spec.

Do you mind explaining in greater detail what exactly was wrong with the bolt.

Was it the actual bolt, or was it the one or some of its small parts (ejector, extractor, springs)?