FTRB issue with a 11.5 BCM

[Patrin]

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.

[17K]

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.

[Patrin]

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.

[ggammell]

Before we go about changing this part or that upper, try a bunch of another ammo.

[Leonidas24]

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.

[JediGuy]

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]

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.

Sent from my SM-G981V using Tapatalk

[17K]

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.

[Red*Lion]

So one malfunction in 500 rounds and it is a dire situation? Give it a good clean and as suggested try some other ammo.

[Patrin]

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.