Failure to lock back after last round

[vizsla]

The bolt won't lock back after the last round is fired using GECO target 55gr 223 ammo. It will lock back on American Eagle 55gr 5.56 ammo, after making some changes. No failure to feeds and no failure to extracts.

Setup:
11.5 inch Centurion Arms barrel.
Vltor A5 buffer tube and spring
BCM BCG
Kies Blast Master linear comp.
PMAG's Gen 3

Initial setup was the above with:
Springco Green spring
Vltor H4 buffer
No failure to feed or extract.
Failure to lock back on last round with 223 and 5.56 ammo.

Switched buffer to Vltor H2. 5.56 no issues. 223 failure to lock back on last round.

Removed Springco spring and used Vltor supplied spring.
Vltor H1 weight. 5.56 no issues. 223 failure to lock back.

Placed Failzero BCG full auto in with Vltor spring and H2 weight. 223 no issues.

Just to see, I put my BCM Mid-length upper on the lower with the green Springco spring and Vltor H4 weight and no issues with 223 and 5.56.

I just ran a couple of rounds tonight of the 223 with Failzero BCG. I have a class this weekend and I am concerned about the reliability.
My questions.
1. Has any one else had issues like this and have some insight to offer?
2. Is it possible to shorten the Springco spring or the Vltor spring?
3. Can I use a Springco Blue spring with the Vltor A5 system?
Thank you.

[AKDoug]

There is a lot of information here to digest.

First, switching uppers proves nothing. Forget that your middy runs and your SBR doesn't.

Is your Fail-Zero a semi-auto carrier like the one I have? If so, there is your answer. You are undergassed and the lighter carrier is helping out. If not, well I have no clue.

Answers to your questions..
1) Your setup has issues, lots of people have issues with SBR barrels when they try to run shitty .223 with a barrel designed to shoot milspec 5.56 ammo.
2) Shortening the springs is not the answer.
3) The Vltor A5 uses a rifle length spring. I use Sprinco Greens.

[Clint]

Sounds like the upper may be slightly under gassed.

This is good for 556 ammo or suppressor use.

Verify there are no excessive gas leaks.

Opening up the gas port slightly may be the be final option after ruling out all others potential issues.

[Robb Jensen]

I think it may be over buffered. If you have a standard length CAR receiver extension try it with a standard rate CAR buffer spring and a H buffer.
I also think it could have a gas leak, check gas block alignment, good tight fit between the gas block and barrel. Gas tube for cracks and fit to the carrier key and carrier key to bolt carrier.

[markm]

Christ. It's an aftermarket goat rope going on there. Let me contact some NASA engineers to start running the calculations on all those variables. :confused:

[Fox33]

not to be captain obvious here but what type of magazines and was it the same one? I've seen similar results (on other platforms) caused by the magazines.

[markm]

I guess.... I'd FIRST... without a doubt ditch the rainbow bright springs. I've not heard of them causing problems... but might as well eliminate one variable.

Then.... Lo and behold get the H2 buffer. (probably not available, but you could make one out of the weights of the two buffers you do have)

H4 is over the standard, and H1 is way under the standard weight.

[vizsla]

Thank you for the replies.
I added some details that people are asking about. I did mistakenly say it was the Vltor H1 buffer when it is their standard weight H2 buffer. Sorry about the mistake. I am using Gen 3 Pmags. Both carriers are full auto. The gas block fits on tight. Just eyeballing the alignment of the gas block, there is a couple of degrees tilt to one side.

I will take off the rail tonight and check for any leaks. Also, I will try it with a carbine extension and buffer weights. I will also try using BCM aluminum mags.

[markm]

Ok... cool. The standard H2 is where you should be looking to get the gun running.

[Duffy]

Does it have a battery assist device, or similar, lever?

[bruin]

Switched buffer to Vltor H2. 5.56 no issues. 223 failure to lock back on last round.

Removed Springco spring and used Vltor supplied spring.
Vltor H2 [CORRRECTED] weight. 5.56 no issues. 223 failure to lock back.

Placed Failzero BCG full auto in with Vltor spring and H2 weight. 223 no issues.Let me see if I understand what you're saying in regards to .223 lock back on the last round:

WORKS - Failzero BCG, Vltor spring, A5H2 buffer, M3 PMAGS
DOESN'T WORK - BCM BCG, Vltor spring, A5H2 buffer, M3 PMAGS

Have you lubed all moving parts generously? Is the gas tube aligned and sliding easily into the gas key? Is your BCM BCG broken-in, or have you polished the friction surfaces? The NiB coating on the Failzero might be saving you, or it could be something with the BCM carrier. Also check the gas key for occlusions.

I don't think the issue is with your buffer system as is. You can try both configs with BCM mags to see if that makes any difference.

[bruin]

Switched buffer to Vltor H2. 5.56 no issues. 223 failure to lock back on last round.

Removed Springco spring and used Vltor supplied spring.
Vltor H2 [CORRRECTED] weight. 5.56 no issues. 223 failure to lock back.

Placed Failzero BCG full auto in with Vltor spring and H2 weight. 223 no issues.Let me see if I understand what you're saying in regards to .223 lock back on the last round:

WORKS - Failzero BCG, Vltor spring, A5H2 buffer, M3 PMAGS
DOESN'T WORK - BCM BCG, Vltor spring, A5H2 buffer, M3 PMAGS

Have you lubed all moving parts generously? Is the gas tube aligned and sliding easily into the gas key? Is your BCM BCG broken-in, or have you polished the friction surfaces? The NiB coating on the Failzero might be saving you, or it could be something with the BCM carrier. Also check the gas key for occlusions.

I don't think the issue is with your buffer system as is. You can try both configs with BCM mags to see if that makes any difference.

[Clint]

Thank you for the replies.
I added some details that people are asking about. I did mistakenly say it was the Vltor H1 buffer when it is their standard weight H2 buffer. Sorry about the mistake. I am using Gen 3 Pmags. Both carriers are full auto. The gas block fits on tight. Just eyeballing the alignment of the gas block, there is a couple of degrees tilt to one side.

I will take off the rail tonight and check for any leaks. Also, I will try it with a carbine extension and buffer weights. I will also try using BCM aluminum mags.

The A5H2 should be the most reliable setup.

No need to swap in a CAR extension.

So, as of now with the stock A5H2,
- the BCM BCG doesn't lock with 223
- the FZ BCG does lock with 223

Is that right?

Currently I'm thinking not enough gas with 223 due to leaks or a small gas port

Whatever differences there are between the 2 BCGs is enough to put it over the edge.

**

With the rail off, check all the gas leak areas per Robb.

If you have the ability, try pulling the gas block, checking alignment and measuring the gas port size.

Try a gas ring check on both bolts.

Try swapping the bolts to isolate bolt or carrier.

[Col_Crocs]

Did you install the gasblock yourself or was it factory pinned? Not challenging your competence, just eliminating variables.

[Iraqgunz]

Before you do anything else, and mess around you need to check the port size on the barrel. Once you have done that come back and let us know. Then we can start to nail down.

[vizsla]

There is no battery assist device or lever on the bolt catch.

I did install the gas block myself. It is a BCM low profile gas block. It fits tight on the barrel. When I removed the rail there were no signs of gas escaping from the gas block or gas tube.

I can't accurately measure the port size. I can place a 1/16 drill bit into the port. It's not tight, but not loose either. The drill bit measures 0.061 inches. There is about 5 degrees of movement with the drill bit when fully inserted into the port.
Ok, so I was able to place some hard setting compound in the port. It measures 0.062 at the narrowest and 0.065 at the widest.

Thank you for the help.

[deerndove14]

Christ. It's an aftermarket goat rope going on there. Let me contact some NASA engineers to start running the calculations on all those variables. :confused:

I'm freaking rolling Mark! Do you stay up at night thinking of this!?:lol:

[AKDoug]

That's pretty small. IG will chime in here, but my notes from his class say .076 for a 11.5"

[Iraqgunz]

If that is indeed correct, then yes it is too small and explains the cycling issue.


That's pretty small. IG will chime in here, but my notes from his class say .076 for a 11.5"

[vizsla]

Ok. So did some tweaking. I removed the gas block measured the gas port and tried to do a better job of aligning the port and the gas block. Everything was well lubed with weapon shield and FP-10

Testing. Kept the rail off.
1. Started off with a different BCG, BCM full auto. H2 buffer, Vltor spring. 223 no issues. Bolt locked back.

2. Same BCM BCG. H4 buffer, Vltor spring. 223 no isssues. Bolt locked back.

3. Same BCM BCG. H4 buffer. Green springco spring. 223 bolt would not lock back.

4. Original BCM BCG full auto. H2 buffer. Vltor spring. 223 no issues, bolt would lock back. Hooray!

5. Original BCM BCG. H4 buffer. Vltor spring. 223 bolt would not lock back.

6. Original BCM BCG. But put in a new bolt. H4 buffer. Vltor spring. 223 bolt would not lock back.

Analysis.
My misalignment of the gas block did contribute to the failure for the bolt locking back. Is there a better way to do this than eyeballing it? The dimple placed on the barrel does not match where the gas block should be lined up. I had the same thing on another Centurion barrel.

Something is going on with the "original" BCG since the 2nd BCG works with the H4 buffer. It didn't look like any gas was escaping under the key. It is staked well.

Plan.
I will just use the 2nd BCG, H2 buffer and Vltor spring. Unless any other issues arise with this setup.

I appreciate all the help given. Thank you all, very much!

[Clint]

That's pretty small. IG will chime in here, but my notes from his class say .076 for a 11.5"


If that is indeed correct, then yes it is too small and explains the cycling issue.

Mine too!

A .073 would probably also work.

Here is a vid of MARKM rocking an 11.5 with a 073 equivalent port
http://www.youtube.com/watch?feature=player_embedded&v=MADmpVSBJVc#action=share

If the barrel were mine, i'd buy .070, .073 and .076 drill bits, then start with the smallest and verify function with a good reliability margin.

When it runs with a weak hold using 223, call it good.

[Iraqgunz]

I know for a fact that .076 will allow that to work in suppressed and unsuppressed mode and it will cycle with a wide variety of ammo.


Mine too!

A .073 would probably also work.

Here is a vid of MARKM rocking an 11.5 with a 073 equivalent port
http://www.youtube.com/watch?feature=player_embedded&v=MADmpVSBJVc#action=share

If the barrel were mine, i'd buy .070, .073 and .076 drill bits, then start with the smallest and verify function with a good reliability margin.

When it runs with a weak hold using 223, call it good.

[wetidlerjr]

...Here is a vid of MARKM rocking an 11.5 with a 073 equivalent port
http://www.youtube.com/watch?feature=player_embedded&v=MADmpVSBJVc#action=share...

Great vid. I see they all ejected the same.
(Bill looks for pie chart so markm can assure himself that all is well with his ejection pattern.)















:smile:

[Iraqgunz]

Centurion Arms has been advised about this and they have advised that some barrels have port issues. Please see your PM's and contact him about the problem.