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Bolt Carrier Locking On Mag Catch

11K views 78 replies 18 participants last post by  Equalizer_2 
#1 · (Edited)
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! :a21:
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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#7 ·
You're impression is correct.
I don't recall swapping springs in lowers today. The only spring experimenting I know I've tried on a previous outing is using one of those highly polished springs and it is 11.25". It didn't make a difference either
 
#10 ·
My thoughts

It might be over gassed a little and the extra .25 spring length is helping it run correctly. If you have access to an H2 buffer I would try that and see if it runs. I always use an adjustable gas block on all my builds so I can tune the gas systems and not screw around with changing buffers.
 
#13 ·
The buffer tube on your 6.8 is a mil spec, shorter tube, thus the spring tension will be slightly higher. Your other lower has a commercial spec tube on it that's longer thus less spring tension. Your solution is probably a low power Carbine buffer spring in the mil spec tube or switch to a commercial tube. Actually, the real solution would be more gas. Your rifle is right on the edge of under gassed. Even that very slight extra bit of spring tension is causing it to short stroke and shouldn't. I hate to ask, but are these 20" or plus rifle gassed 6.8's? If so, they can be very sensitive to anything that holds back the carrier even a little bit.
 
#14 ·
Both rifles are nearly identical. I'll list the parts.

80% Arms Upper
80% Arms Lower
ARP 18" Midweight 6.8 SPC barrel
M-16 Bolt Carrier
6.8 9310 Bolt
Troy Gas Block
Linear muzzle device
BCM lower parts kit
BCM Gunfighter stock, carbine buffer spring, end plate. These came packaged.
ASC magazine
BCM charging handle
One rifle has BCM keymod forend and the other has a Lancer carbon fiber forend
Vortex Diamondback HP 3-12x scope

More gas huh? So mill out the gas port on the barrel? I can't get any more gas in the system any other way, unless I load even more hot than I am. I took the time to open the gas tube port to match the gas block and the gas port on the barrel is within spec according to info provided by H.

Or try a reduced power spring from Wolff?

What about cutting a coil off the carbine spring?
 
#15 ·
Have u tried it with factory hornady SST or factory s&b ammo? Or just your reloads?

Is your 18" barrel mid gas or rifle gas length? (Maybe I missed that part)

What powder and how much in your reloads.

If you are slightly under gassed you may need to upended the gas port by 1 bit size
 
#17 ·
Have u tried it with factory hornady SST or factory s&b ammo? Or just your reloads?

Is your 18" barrel mid gas or rifle gas length? (Maybe I missed that part)

What powder and how much in your reloads.

If you are slightly under gassed you may need to upended the gas port by 1 bit size
ARP 18" midweight with midlength gas.
I have shot factory Hornady match 2670 fps, and factory SSB and it does the same thing. My loads are 29.6gr of AA2200

How many rounds have been fired through these guns?
Reason I ask is: with a little break in they will probably start locking back all the way if they haven't been shot much.

Will they reliably feed more than 1 round from the mag? If so, shoot them.
Yes, they both cycle semi-auto reliably. One of them is about a year old and has over a couple hundred round through it. The other is a couple months or less with less than 200 rounds.

They are both past a couple boxes of ammo, I just don't record the number of rounds.
 
#18 ·
Springs are cheap. The BCM springs may simply be too heavy. It's not a coincidence both rifles are short stroking with the same buffer tube, buffer and springs in them. Just because they are name brand doesn't mean they can't be out of spec. You'd be surprised how many times that's the case and how many times a simple buffer spring change can make a short stroking rifle function. I once put what they call a "Twang Stopper" in my tube that was advertised to lessen the operating noise. It was about 3/16" thick. Immediately the rifle started short stroking. Took it back out and it's functioned perfect since. That little extra bit of spring compression was all it took. One of my AR10 lowers caused both my uppers to SS. I had bought a cheap mil spec buffer system for it and it was very hard to hand cycle or charge. I put a DPMS spring in it and functioned perfectly and was definately easier to hand cycle and charge.
 
#19 ·
Hmmm.

Looks like a have a number of options and porting the barrel more should be the last resort since that's permanent.

Yes, springs are cheap. I was expecting more out of BCM kits. Stocks are fine and small parts seem to be fine but, I think I might pass on the springs, tube, and trigger for the next builds.

Or perhaps it's all in my mind and not in reality that ARs should be easier than 1911s to trouble shoot. /rantoff

While they both shoot just fine, it's become a matter of personal pride to get them both 100%. The time spent milling, building, load development, sighting in, refinishing, trouble shooting, all to realize, I'm really at 95%. I really want to get that last 5%. One of these rifles is for my YL and her first season of hunting this fall.
 
#20 ·
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.
 
#24 ·
This may sound crazy but have you compared the bolt catch itself? I had the same problem with an aftermarket catch that was slightly taller and more squared off in the front than a standard one. I took a file to it and it fixed the problem.
 
#25 ·
It doesn't sound crazy at all. In fact I miced the height of the catch from the top of both the 300 and 6.8 receivers and found them to be the same height. What I didn't check was geometry. It could be just out of spec enough in one way or another. I'm currently stuck, mentally on the follower spring being too slow. Which brings me back to the comment of it being over gassed. If it's over gassed, would it be forcing the BCG back so fast that the buffer spring is returning it equally too fast for the follower spring to push the follower up fast enough to catch the bolt?
 
#28 · (Edited)
Perfect ejection but not locking back on the bolt. I would try a different bolt catch, something fishy going on that it'll pick up new rounds but won't lock back on the bolt face. Somebody else might chime in, its been a while since I've assembled a lower, I believe the bolt catch spring is directional where the larger end goes into the receiver and smaller end has the little nub in it riding against the bolt catch. If the spring is backwards it'll catch in the hole and fight the spring tension of the magazine follower.

I did this in one of the first few lowers I built, the catch was super sticky and wouldn't spring back down after pushing the button to raise it up. Had to push the roll pin back carefully and change the spring around, I don't especially like those bolt catch roll pins.
 
#38 ·
Nothing fishy. The brass ejects a long way before the bolt gets back to the bolt catch. In fact, as soon as the empty case neck clears the chamber, well before it reaches the point of catching on the follower. I had a rifle you could shoot all day and it would eject and feed all day. But it still short stroked enough, before I drilled the gas port, to never reach the bolt catch and always caught on the follower.
 
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