View Full Version : FTRB issue with a 11.5 BCM
Have identical setups : Trainer 11.5 and Primary 11.5 (BCM) on Colt lowers. Only difference, the primary running an NP3 Sionics BCG. Round counts respectively, 10K+ trainer...2K on Primary. No suppressor use.
Traditionally, both have been run w/ H2's and standard springs. The trainer has been stupid reliable from cheap steel to hot 5.56. Primary has run only Federal and IMI 5.56.
Early on, the primary had issues around 500 rds w/ FTRB and not holding a case. The issue wouldn't develop till deep into a training session. I suspected a tight chamber. Sent to BCM to resolve. BCM returned with note to run an o-ring and H3 buffer. The trainer always setting the baseline, no o-ring, never an issue.
So I popped o-rings in both rigs, kept H2's and changed the sionics bolt to a spare new BCM bolt to eliminate much as possible. Both are running good.
In the last 6 months, switched to A5's w/ A5H2's on Colt rifle springs. Again, the trainer is running no issues. First session w/ primary (300 rds) is uneventful, a second this weekend, at around count 200, I get a FTRB, but holding the case, using the FA to close, another 100 no issues. Ammo is IMI M193.
I'm at a loss. There's a good enough track record with A5's on 11.5 BCM's to know that's not the issue. +, the recoil impulse is much improved with the A5's.
I'm thinking the chamber is just a smidge tighter on the primary, and after some crud gets in there, voila.
Other thoughts?
ETA : in addition to o-rings, also running Colt gold extractor springs.
I would swap the primary upper for a Colt 6933 upper.
It will run with the A5, no o-ring, and your ammo because it won’t be grossly overgassed and have an out of spec chamber.
I went through this EXACT scenario a couple of months back with an 11.5” that belongs to a guy that works for me.
That's certainly one way to go - but in this market...$$$.
Next steps I had in mind was to polish the barrel extension/ramps (check for burs), try a springco green spring and the Vltor equivalent weight of an H3.
...before dumping it that is.
Before we go about changing this part or that upper, try a bunch of another ammo.
Leonidas24
08-17-21, 22:49
I would swap the primary upper for a Colt 6933 upper.
It will run with the A5, no o-ring, and your ammo because it won’t be grossly overgassed and have an out of spec chamber.
I went through this EXACT scenario a couple of months back with an 11.5” that belongs to a guy that works for me.
This, more or less. Those uppers are stupid overgassed which is why BCM told you to run an H3 and o-ring. A properly gassed upper will run on an H/H2 and no o-ring. Regarding the Vltor A5s, it wouldn't come as a surprise to hear that those barrels work fine with A5H4s and Sprinco green springs.
Clint should be along shortly to recommend one of his gas tubes and rightly so. I like BCM for the most part but having a gas port larger than a Crane spec 10.3 on a longer barrel baffles the mind.
Was this a shift/When did the shift to BCM’s larger gas ports happen? I remember IG recommending a heavier A5 buffer for a 12.5 when I first got into this stuff. At the same time...it seems like their 14.5 midlength was the bees knees back in the day. What happened (if anything)?
Leonidas24
08-17-21, 23:38
Was this a shift/When did the shift to BCM’s larger gas ports happen? I remember IG recommending a heavier A5 buffer for a 12.5 when I first got into this stuff. At the same time...it seems like their 14.5 midlength was the bees knees back in the day. What happened (if anything)?I don't think there ever was a "shift." The earliest report of gas port size on a BCM 11.5 dates back to 2009 and has fluctuated between .073-.076 since.
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Their barrels in general just aren’t that great.
I had a pair of 16” mids that cycled so hard they beat up firing pin retainer pins every couple hundred rounds, a 14.5” SOCOM that pulled the rim off of several cases, and a 16” carbine that popped primers showed pressure signs with ammo that was perfectly fine with LE6920s.
Not to mention the lower that had the buffer retainer pin hole mislocated and tore up buffers, and the lower that wouldn’t accept Gen M3 Pmags.
So one malfunction in 500 rounds and it is a dire situation? Give it a good clean and as suggested try some other ammo.
So one malfunction in 500 rounds and it is a dire situation? Give it a good clean and as suggested try some other ammo.
Certainly not dire, seems like a pattern though with the upper. Ammo is not the issue, nor is cleanliness.
The comments on over gassing I understand. When purchasing the trainer upper, the first, I didn't mind it being over gassed, and it didn't seem like much. I live in a cold climate and having
the upper function everything I feed it is a plus.
The primary, second upper, must be 'more' over gassed than the trainer. Maybe the trainer is .073, and the primary .076. Purchased a year apart, different batch, I don't know.
Thank you for the thoughts.
So one malfunction in 500 rounds and it is a dire situation? Give it a good clean and as suggested try some other ammo.
If it won’t make 500 rounds of Federal and IMI 5.56, something is wrong.
If he has to clean it and start picking through ammo to find something that runs, something is wrong.
I wouldn’t waste another second or penny on it.
If it won’t make 500 rounds of Federal and IMI 5.56, something is wrong.
If he has to clean it and start picking through ammo to find something that runs, something is wrong.
I wouldn’t waste another second or penny on it.
You assume that all 500 rounds were in spec. Which, as we all should know, occasionally they aren’t.
Used to be, around here, we would try a few minor things, get a better feel for what’s going on. Now we’re at ditch half the gun as first step.
I miss the good old days.
You assume that all 500 rounds were in spec. Which, as we all should know, occasionally they aren’t.
Used to be, around here, we would try a few minor things, get a better feel for what’s going on. Now we’re at ditch half the gun as first step.
I miss the good old days.
I'm tracking with your suggestions. Gonna re-spring first. Failing that, a BRT tube (suggested). Upper has a good, accurate barrel on it...is zeroed with my irons, RDS and thermal.
Not gonna shit can it so easily.
You assume that all 500 rounds were in spec. Which, as we all should know, occasionally they aren’t.
Used to be, around here, we would try a few minor things, get a better feel for what’s going on. Now we’re at ditch half the gun as first step.
I miss the good old days.
Those barrels are a known quantity/quality and from experience, I wouldn’t bother.
With the cost and availability of good ammo, and the scarcity of time, a new barrel is cheap.
Those barrels are a known quantity/quality and from experience, I wouldn’t bother.
With the cost and availability of good ammo, and the scarcity of time, a new barrel is cheap.
Until you have to pay someone to switch it.
And this is the first time I’ve heard that bcm 11.5s are so horribly over gassed.
I am having trouble understanding why an overgassed rifle would fail to return to battery. Am I misunderstanding the acronym?
Andy
I am having trouble understanding why an overgassed rifle would fail to return to battery. Am I misunderstanding the acronym?
Andy
It makes no sense to me either.
One FTRB…..so scrap the upper.
That’s hilarious.
I don't understand how "horribly over gassed send it to the dust bin" equates to what is essentially a 2% failure rate (roughly 1 FTRB every 500 rounds?) Are all the mags you are using of the same type and manufacturer?
Gen 3 pmags, newish NHMTGs, refurbished NHMTGs. All aluminum have either sprinco or SAW red springs w/ magpul followers.
Upper is not getting shit canned. Green spring from sprinco is inbound and I will update the thread after next range sesh.
I don't understand how "horribly over gassed send it to the dust bin" equates to what is essentially a 2% failure rate (roughly 1 FTRB every 500 rounds?) Are all the mags you are using of the same type and manufacturer?
That's a 0.2% failure rate. Higher than I'd feel comfortable with, given quality ammo, but not a crazy rate.
Gen 3 pmags, newish NHMTGs, refurbished NHMTGs. All aluminum have either sprinco or SAW red springs w/ magpul followers.
Upper is not getting shit canned. Green spring from sprinco is inbound and I will update the thread after next range sesh.
Yeah I didn't think you were shit canning it based on your replies. Just laughing at the folks who were telling you to when a $20 spring could correct the issue.
Sounds like you can rule out the mags. Was just curious since no one had asked until now. Look forward to your range report.
That's a 0.2% failure rate. Higher than I'd feel comfortable with, given quality ammo, but not a crazy rate.
Dude. Don't make me do math that requires a decimal point prior to having caffeine. That's my story and I'm sticking to it.....
ETA : in addition to o-rings, also running Colt gold extractor springs.
Get rid of the O rings. Colt springs are designed to give proper tension on the extractor without an O ring. You might have too much tension on the extractor, preventing it from snapping over the case rim.
Get rid of the O rings. Colt springs are designed to give proper tension on the extractor without an O ring. You might have too much tension on the extractor, preventing it from snapping over the case rim.
This sounds likely. I would try this first.
*If* the barrel is so terribly overgassed then it’s possible that what appears to be FTRB is actually bolt bounce where the BCG is slamming home so hard it comes out of battery, but this usually only happens on full auto.
What happens when you put the primary upper on the training lower? Do you have another lower with a carbine RE?
I don’t think the issue is caused by the lower, but if it’s easy to take it out of the equation, then it’s worth doing. It sounds like the FTRBs only occur when the chamber is dirty, though it’s been sent to BCM, and they say the chamber is GTG, but it could still be too tight (I haven’t read of any instances of BCMs being tight enough to induce malfs).
Failure To Return to Battery also occurs when the action spring is weak.
A few things to consider, and my guess is that you have recently given your weapon a GOOD cleaning and that it is properly lubed. With those primary issues being out of the way, the next is #3.
1. Dirty weapon
2. Insufficient lube
3. Weak main action spring
4. Weak ammunition
5. Loose carrier key
6. Misaligned gas tube
7. Leaking gas block
8. Weakened / loose bolt rings
9. Bad magazines
What happens when you put the primary upper on the training lower? Do you have another lower with a carbine RE?
I don’t think the issue is caused by the lower, but if it’s easy to take it out of the equation, then it’s worth doing. It sounds like the FTRBs only occur when the chamber is dirty, though it’s been sent to BCM, and they say the chamber is GTG, but it could still be too tight (I haven’t read of any instances of BCMs being tight enough to induce malfs).
I changed both to A5's. The trainer gets changes first for vetting and if all goes well for a few months, I make the change to the primary and then test it.
Another note to when the upper was at BCM, it was run in full auto and the gas tube was changed prophylactically.
To MistWolf, I get the feeling the Colt rifle spring doesn't have enough strength (it's a new spring). It's my understanding the green Sprinco has a bit more punch, and is on the way.
To Stickman, of the list, I've eliminated all except #7 and will eliminate #3 soon.
I’d do a good PM cleaning and inspection on the trainer and make it the primary, put the primary into the training role and see what happens.
I’d do a good PM cleaning and inspection on the trainer and make it the primary, put the primary into the training role and see what happens.
Yep, we're on the same page there. Primary is the trainer for the duration till I'm satisfied with performance.
Sorry for the late update gents, wanted to get a couple training sessions in before updating.
150rds per sesh, no FTRB or malfunctions of any kind.
After exhausting the list Stickman put forth over the course of time, the only change made to address this issue, now, was changing to a Sprinco 'GREEN' spring from the standard COLT rifle spring.
Will update again after a few more outings.
I get the feeling the Colt rifle spring doesn't have enough strength (it's a new spring). It's my understanding the green Sprinco has a bit more punch, and is on the way.
I don't think the green spring has more "punch" but I do believe it's more consistent.
I’m thinking that the A5 reliability window may not as big as we thought it was as finicky as they are about springs.
I’m thinking that the A5 reliability window may not as big as we thought it was as finicky as they are about springs.
If a spring is bad, it’s bad.
Green springs have the same spring rate as a standard rifle spring.
If a spring is bad, it’s bad.
Green springs have the same spring rate as a standard rifle spring.Could there be other, less obvious factors? Such as friction inside the receiver extension being higher with one spring vs the other, for example.
Could there be other, less obvious factors? Such as friction inside the receiver extension being higher with one spring vs the other, for example.
It's possible there could be quality control issues or some part being out of spec.
The A5 was thoroughly tested- by the Marine Corps, I believe. They found it more reliable than the rifle RE assembly which is pretty damn reliable.
The AR is a very reliable, durable and mature design. When everything is right, when everything is in spec, it works every time. If it doesn't work, something is out of spec.
If an AR with an A5 RE assembly has a narrower span of operation than normal using a properly assembled A5 buffer, the problem is likely to be an out of spec spring. That doesn't mean the A5 assembly is spring sensitive as much as it means the AR is spring sensitive. It needs springs made to spec.
Look at the following malfunctions and the cause-
-Extraction malfunctions are caused by out of spec extractor springs
-Weak ejection is caused by out of spec extractor springs and/or out of spec ejector springs
-Incomplete return to battery is caused by out of spec action springs
-Weak ignition and light primer strikes are caused by out of spec hammer springs
-Feed issues can be caused by out of spec mag springs
-Trigger malfunctions can be caused by out of spec trigger return and/or semi-auto block spring
Without springs any firearm becomes useless. Too many AR owners downplay the importance of spring specifications and quality. When ARs have about a thousand rounds through them, especially when several hundred rounds are fired in a day, springs that aren't up to snuff start making themselves known. I found that out first hand and it wasn't my Colt or Sprinco springs that were failing. It was the generic springs.
Once I have an AR upper properly sorted out, that is the gas drive is right (correct gas port, all leaks fixed) and extraction & ejection problems eliminated (Colt extractor spring, ejector checked for proper function) I find it works with any lower with any in spec buffer & spring (carbine RE with standard Colt spring and H2 buffer, rifle RE assembly with quality rifle spring and rifle buffer and A5 RE assembly with A5H2 buffer & Sprinco green spring).
Uppers include a 20 inch, 16 inch middy, 16 inch carbine, 14.5 inch carbine and 11.5 carbine with and without suppressor. The shorty uppers are set up to be always shot suppressed, or always shot unsuppressed. I shoot mostly XM193 except in the precision ARs.
With one exception, I've never had to adjust the gas on any of my Colt uppers. The exception is a 6933 set up as a suppressor host.
^I’ve long held an idea that maybe, on a semi-auto, the buffer almost doesn’t matter as long as the upper is good to go.
But there sure has always been a lot of chatter about buffers. Maybe it’s because they’re so easy to change.
ViniVidivici
09-09-21, 00:09
Naw, there IS a difference.
My guns run smoother, and are less jumpy under recoil, with H2 buffers as opposed to standard or H.
Many other folks have also had this actual, physical experience.
It's possible there could be quality control issues or some part being out of spec.
The A5 was thoroughly tested- by the Marine Corps, I believe. They found it more reliable than the rifle RE assembly which is pretty damn reliable.
.
In that test the carbine spring/H6 equipped guns also outperformed the A4, equaled the A5 on reliability, and the A5 had a failure to lock back, when the H6 didn’t. The stoppages that occurred with the A5 and H6 equipped guns were all bad mags and light strikes except for the one failure to lock back with A5. Extrapolate what you will from that.
66416
From my experiences, I don’t think the A5 performs as well as well as a carbine/H or H2.
The green spring does have a slightly higher force than a regular rifle spring.
Something like 4-5% more.
I don't think the green spring has more "punch" but I do believe it's more consistent.
In that test the carbine spring/H6 equipped guns also outperformed the A4, equaled the A5 on reliability, and the A5 had a failure to lock back, when the H6 didn’t. The stoppages that occurred with the A5 and H6 equipped guns were all bad mags and light strikes except for the one failure to lock back with A5. Extrapolate what you will from that.
66416
From my experiences, I don’t think the A5 performs as well as well as a carbine/H or H2.
As the H6 buffer (5.2 oz) is about the same weight as a rifle buffer (5 oz), it's performance shouldn't come as a big surprise. I haven't tested the H6 or the H3 (5.4 oz). I don't have any on hand.
Recoil was softer with the A5 with green spring and A5H2 buffer than the H buffer with standard carbine spring with 11.5 inch suppressed uppers. Recoil with the H2 buffer was almost as soft as with the A5H2 combo.
The softest and flattest recoiling combo I tested was the A5H2 and green spring with a 20 inch HBar profile barrel and Battlecomp. The reticle hardly moved at all during recoil.
The green spring does have a slightly higher force than a regular rifle spring.
Something like 4-5% more.
Thanks for the info.
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