LRBHO Failure - Stumped

[Hansel&Grendel]

Finished a 6.5 Grendel 11" upper with a Superlative Arms (SA) adjustable gas block for a former work buddy. The lower receiver was delayed, so I tried to adjust the gas block for proper bleed off with an almost identical lower from an AR 7.62x39 build that has an Aero Precision lower, a SBA3 pistol brace, a Spikes Tactical ST-T1 3 oz (carbine) buffer, a JP Tuned Carbine Buffer Spring, a Strike Industries enhanced bolt catch, and a Law Tactical Folding Stock Adapter.

I set the SA gas block at 4.5 CCW turns from closed (maximum gas). I could not achieve LRBHO from maximum gas and no matter how I adjusted the SA gas block. However, the AR 7.62x39 lower does achieve LRBHO every time with its dedicated upper using CPD 28rd AR 7.62x39 mags.

I then took a NFA lower from a 6.5 Grendel SBR build that has a Mega Arms lower, a DS Arms 3 oz carbine (steel weights) buffer, a POF anti-tilt buffer tube, a JP Tuned Carbine Buffer Spring, a POF enhanced bolt catch, and also a Law Folding Stock Adapter. LRBHO with the new 6.5 Grendel upper was no problem from maximum gas and no problem after I found the "sweet spot" bleed-off position for the SA gas block. Several rounds confirmed this.

I built the new lower this weekend with the more robust POF enhanced bolt catch (larger tang). However, once again, I could not achieve LRBHO. The new lower is identical to the aforementioned AR 7.62x39 except for a Spikes Tactical stripped lower.

I'm stumped with what's causing the LRBHO failure -- the bolt is either not captured or is barely captured because when I release the empty mags, the bolt flies forward. The mags used are the newly designed CPD 6.5 Grendel 26rd mags. BHO is achieved with all 3 lowers when working the charging handle with the new mags empty.

The Toolcraft 6.5 Grendel bolt does not appear to be at fault because it works flawlessly (including LRBHO) when paired with the NFA lower described above. What appears to be common with the LRBHO failure is a SBA3 pistol brace and a ST-T1 carbine buffer (tungsten filled powder vs. steel weights). What's interesting is that after symmetrically lining up each JP Tuned Carbine Buffer Spring, the number of coils is 34 for the new build, 37 for the AR 7.62x39 build, and 36 for the NFA 6.5 Grendel build (the one that achieves LRBHO). Not sure if the number of coils makes a difference.

Any ideas on what's causing LRBHO failure? I have a Sprinco "White" carbine buffer spring that's been sitting on the shelf that I can try. Beyond that, I'm stumped.

Thanks! Hans

[A5BLASTER]

Most likely bolt catch drag.

Put in a cheap and standard bolt catch and see what it does.

[montana]

Pull the charging handle back and measure how far back the bolt goes in the receiver that works. Do the same on the receiver that won't work and see if there is a difference. If there is , pull the recoil springs and see if their is a difference in their length. Receiver tubes do not always have the same internal length, which is why I use Viltor mil spec receiver tubes exclusively. It sounds like the combination of recoil spring, buffer, and lower receiver tube works, so find the difference. Change the bolt carrier catch with the receiver that works, etc, process of elimination.

[Hansel&Grendel]

The bolt appears to go back equal distance with each of the 3 buffer springs with the charging handle pulled all the way back. The spring from the NFA build, even with its 36 counted coils is about 3/8" shorter than the new spring with its 34 counted coils, but likely attributable to spring compression from the number of rounds fired through the NFA build. The Sprinco "White" carbine spring has 31 counted coils and is about 3/4" shorter than the spring from the NFA build; it's the NFA build buffer spring that results in LRBHO but in a different lower. Not sure how many rounds have been fired with the Sprinco since it's been sitting on the shelf for a while.

What I will try at the range is the NFA build spring in the new lower as well as the Sprinco "White" spring in the new lower. At this time, I'm inclined to think that it's not so much the bolt catch as I've never had a problem with either of them (Strike Industries or POF) on all the builds that I've done. But, I'll keep it in mind just in case.

[Shooterrex]

Put the nfa build spring AND buffer in the new build and see if it runs. What size is your gas port?

[VASCAR2]

Generally if the bolt is not captured by the bolt catch and is held back by the magazine follower there is insufficient recoil momentum. The bolt and carrier are not going far enough back to permit the bolt catch to rise and stop the bolts forward momentum. A sticky bolt catch could contribute to the failure to lock back the bolt. I’ve had to polish one bolt catch as it was very sticky and didn’t move freely. Like someone else mentioned I’d also measure the inside depth of the buffer tube.

The solution might be to open the adjustable gas block a little more with that particular lower.

[Hansel&Grendel]

The inside depths of the buffer tubes do differ but is largely inherent to the design of the POF "anti-tilt" buffer tube that cradles (wraps around) the buffer detent. The external OAL of the POF (6-position) buffer tube and the SBA3 are virtually identical. However, the more I look at this, the differing internal architecture between the buffer tubes may very well be the issue given that all else is pretty much the same.

Granted, the AR 7.62x39 lower with the SBA3 pistol brace/receiver extension combo works flawlessly including LRBHO with its dedicated upper, but not with the 6.5 Grendel upper. Gas pressures are obviously different. So, I'll order another POF anti-tilt buffer tube that I can use for another build if it isn't the solution; and I'll play around with the various buffer/buffer spring combos.

The differing bolt catches just don't seem to be at issue but I'll eliminate any potential issue as I go along.

It's an intriguing issue and a sincere thanks for all the responses.

UPDATE: Replaced the buffer tube with an Aero Precision enhanced buffer tube. Still, no joy for LRBHO. Replaced the ST-T1 and JP Tuned Carbon Buffer Spring with a DS Arms 3 oz carbine (steel weights) buffer and another JP spring (mentioned in the original post) and was able to achieve LRBHO. I then found the sweet spot (bleed-off) for the Superlative Arms AGB. So, it appears that the issue was with either the ST-T1 or the JP spring. The good news is that it's working as it should for LRBHO. Happy end user who picked it up today.