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 Bolt Carrier Locking On Mag Catch 
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We've got two 6.8's in the house and both have been doing the same thing. I didn't notice the first one doing it until I built the second one and was doing a more thorough function check on it. So here's what going on. Both rifles lock back on an empty magazine but, they lock on the carrier and mag catch. Not the bolt and mag catch. I'm clocking 95gr projectiles at over 2800fps.
Carbine buffer and carbine springs. Mid length gas system. I even took a deer last year with one of them and it doesn't suffer any apparent losses in accuracy.
So here's what I've tried so far:

Lubed buffer spring and buffer extension
Checked gas rings
Checked gas block alignment
Ported gas tube to match diameter of gas block.
Checked gas port in barrel.
Sealed the gas tube to the block and gas block to the barrel with gasket sealer.
Changed buffer weight down to 2.2oz
Swapped bolt carrier from a known working firearm

None of that made a difference.

So I took one of the 6.8 uppers and put it on my 300blk lower. The same firearm that was the donor bolt carrier for test. I even left the carbine buffer in it. BANG. Lock on bolt! :bigsmile:
OK so WhAT is your major malfuction???

I'm out of ideas and internet searches aren't giving me any insight. The only thing I can think of now to try is, turning the buffer tube back out a turn or two.

Anyone got any ideas?


EDIT: I just went to back out the buffer extension by one turn and the buffer retainer was free to come out. So that's not going to work either.

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Sun Apr 23, 2017 6:57 pm
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What happens when you slowly manually cycle the carrier on an empty mag? Is the mag not seated high enough to push the catch up all the way? Do you have anything on the catch, like a BAD lever etc that could be causing resistance?


Mon Apr 24, 2017 5:52 am
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No BAD lever. The catch works when you manually cycle the bolt carrier group. It also works when you drop the mag after the last shot. But it's caught on the carrier not the lugs of the bolt.

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Mon Apr 24, 2017 7:17 am
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Is the BCG able to travel back as far in the 6.8 lower as it does in the 300blk lower?
Could the BCG be outrunning the mag spring in the 6.8 mags?
Have you compared spring and buffer lengths?

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Mon Apr 24, 2017 9:16 am
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1811GNR wrote:
Is the BCG able to travel back as far in the 6.8 lower as it does in the 300blk lower?
Could the BCG be outrunning the mag spring in the 6.8 mags?
Have you compared spring and buffer lengths?

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Yes. It will function as it should when it is cycled by hand. I can pull the charging handle back and the bolt lugs are caught by the catch. It does that in both lowers.

Yes, I compared the springs and buffer lengths last night after the original post.
The 6.8:
Spring is 10.25"
Tube is 7.125"

The 300blk:
Spring is 10.5"
Tube is 7.25"

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Mon Apr 24, 2017 9:44 am
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Thinking about physics....I would need a helper to shoot video in slo-mo to verify but, what if it's not short stroking at all? What if the follower spring is too slow (not enough tension) and the buffer spring is ahead of (faster than) the follower spring. It could be that the follower is not coming up fast enough for the bolt to catch.

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Mon Apr 24, 2017 9:53 am
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dreadi wrote:
Thinking about physics....I would need a helper to shoot video in slo-mo to verify but, what if it's not short stroking at all? What if the follower spring is too slow (not enough tension) and the buffer spring is ahead of (faster than) the follower spring. It could be that the follower is not coming up fast enough for the bolt to catch.

That is what I was trying to say with: "Could the BCG be outrunning the mag spring in the 6.8 mags?" Wasn't very clear was it.

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Mon Apr 24, 2017 2:58 pm
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1811GNR wrote:
dreadi wrote:
Thinking about physics....I would need a helper to shoot video in slo-mo to verify but, what if it's not short stroking at all? What if the follower spring is too slow (not enough tension) and the buffer spring is ahead of (faster than) the follower spring. It could be that the follower is not coming up fast enough for the bolt to catch.

That is what I was trying to say with: "Could the BCG be outrunning the mag spring in the 6.8 mags?" Wasn't very clear was it.

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Earlier today I couldn't figure out where I got that idea from. Didn't mean to steal your thunder. Thank you.

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Mon Apr 24, 2017 6:45 pm
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dreadi wrote:
So I took one of the 6.8 uppers and put it on my 300blk lower. The same firearm that was the donor bolt carrier for test. I even left the carbine buffer in it. BANG. Lock on bolt! :bigsmile:


I was going to guess a nasty bolt rough bolt, so that kills that idea.

Never had a lower give me any trouble so I am in guess land right now.

What trigger and hammer group are you using? I mean it's possible but not very probable the hammer (spring) is using up a bunch of the energy.

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Mon Apr 24, 2017 7:18 pm
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Or somehow the bolt carrier is illicitly contacting the inside of the tube, or the buffer is. Some kind of lower misalignment.

A real clue is they both do it. I think comparing them to a running AR would be wise.

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Mon Apr 24, 2017 8:10 pm
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Compare in what ways? Oh you mean to another 6.8.

Trigger and LPK are BCM

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Mon Apr 24, 2017 10:01 pm
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dreadi wrote:
Compare in what ways? Oh you mean to another 6.8.

Trigger and LPK are BCM


What is different about the two lowers that don't work and your .300 that does work?

Do your 6.8 lowers work with your 5.56 and .300 uppers?

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Tue Apr 25, 2017 3:13 am
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The 6.8 lower works with the 5.56 upper.

I can't put the 300blk upper on any other lower because it creates an SBR.

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Tue Apr 25, 2017 7:14 am
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dreadi wrote:
The 6.8 lower works with the 5.56 upper.



So there is some interference in just your 6.8 combos. Interesting. Just side by side compare to your 5.56 with mics and calipers all sliding and contact areas. Sherlock eyeballs on. Something is hanging, the tips above about mag fits and such could be spot on.

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Tue Apr 25, 2017 7:30 am
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What a drag. :cussing:
Have you tried multiple mags from different manufacturers?

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Tue Apr 25, 2017 7:52 am
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