**UPDATE**Request help to diagnose a bad malfunction

[Serpico1985]

Buddy of mine built a 11.5” pistol and has run into problems when shooting his duty/home defense ammo through the gun. The failures seem quite severe and I don’t have a lot of experience with diagnosing AR15 failures so I was hoping someone has seen this before and may be familiar with it.

Quick summary:
He’s put 300 rounds of practice ammo through the gun without issue. The practice ammo is federal 55gr boat tail hollow point:
https://i.ibb.co/8x84Lz1/practice-ammo.jpg (https://ibb.co/R6H4khR)

When he tried running some of his duty ammo through the gun it was choking badly failing to chamber almost every other round. This occurred when he was using gen 2 pmags and 20 round colt mags. When he switched to 30 round GI aluminum mags, it fed. Duty ammo is:
https://i.ibb.co/fkNM9dy/duty-ammo.jpg (https://ibb.co/jywhzkQ)

He didn’t take any pics of the malfunctions when it happened but kept a couple of the rounds that failed to chamber. They look like this:
https://i.ibb.co/t4xSdHq/dents.jpg (https://ibb.co/1Ld3S0r)

The dents match up to the lugs on the bolt.

On the opposite side of the dents there is a deep gouge/scratch:
https://i.ibb.co/p1Zn84G/scratch-oppiste-of-dents.jpg (https://ibb.co/zmbNYPj)

He described the orientation of the round in the upper when the failures occurred like this:
https://i.ibb.co/r7YdSR9/orientation.jpg (https://ibb.co/3r2Rq9K)

All parts used in build are new. He and I put it together. I’m built 7 rifles without issue. Used correct tools etc.

ADM Upper
Radian ambi lower
Geissele Super 42 buffer and spring in a milspec tube
LMT Enhanced complete BCG
Colt 11.5” FBI HRT barrel purchased from Shark Arms

When looking at his barrel extension in the upper I noticed that the transition from upper receiver feed ramps to the barrel extension ramps was not smooth like on all of the previous builds I’ve done. There is a definite ledge on each side and it appears worse on the left side.

Here’s what his upper/barrel extension looks like:
https://i.ibb.co/wpzzHSG/problem-upper-2.jpg (https://ibb.co/1RmmVry)

https://i.ibb.co/gDtCphp/problem-upper-3.jpg (https://ibb.co/P518n0n)


I have no idea if the barrel extension/upper fit is part of the problem, I just noticed it looked different that all of mine. Would love to hear some thoughts if anyone has seen this type of malfunction before. We have access to replacement parts to try and swap stuff out to see if something fixes it (standard bolts, BCG, carbine spring/buffer and a spare unused SOLGW upper).

Thanks for your time.

[lysander]

The exposed lead nose is snubbing on the feed ramp.

[Disciple]

The exposed lead nose is snubbing on the feed ramp.

What can be done about that?

[Serpico1985]

If that were the case wouldn’t it just stop there? How is the BCG getting on top of the round to cause those dents and gouges?

[ggammell]

It’s bumping just enough to slow it down not stop it. The little bump changes the path of the bullet nose just enough that it pops up and the bolt slams into the side of the case instead of pushing on the back of the case.

He’s shooting XM series ammo. That in and of it self is enough to explain the malfunctions. It’s an XM because it didn’t pass contract spec. Could be accuracy. Could be manufacturing defect. I’ve got a pic around here somewhere of those rounds with visibly different OAL (bad manufacturing).

He should move to a factory new, in spec ammo for his home defense ammo. That stuff he has is clearly not viable.

[Pappabear]

Try Gen 3 (20) round Pmags. The Colt 20 rounders have weak springs and I ran into issues with them in one gun. If the 30 rounders are working then you only need to find a 20 round mag that works.

Vltor A5 buffer system might help quite a bit. Do you have a lower with an A5 system in it? Swop that out if you do? I did that and my problems went away, even with Colt mags. My problem was I wasn't getting lock back on last round but I'm such a fan boy of the A5 I had to put the idea out there.

It looks like on the right feed ramp, the rounds are hitting centered and not so much on the left, for whatever reason.

PB

[mark5pt56]

Would check lock back first. Shoot at least 10 singles in two different magazines.

[Serpico1985]

Pappabear,

We have access to A5 equipped lowers and gen3 30 round pmags.


Ggammel,

I'm not familiar with the "XM" designation being second run. I'm not saying I don't believe you but is there some place else I could check to confirm this? Do you think it's purely the ammo or is the feed ramp a problem also?


mark5pt56,

Are you saying check lock back first with the A5 lower?


I'm confused as to how the A5 could fix this issue. More spring pressure with the rifle spring? I don't want to do a bandaid fix if the route of the problem is the barrel ramp or the ammo. I want to fix the problem. I would think that a quality rifle should work with any quality mag and any quality ammo. I'm not second guessing anyone, I just am wondering what you think the problem is exactly that a different buffer/spring and mags would fix it.

Thanks for your input!

Zack

[ggammell]

XM anything from Federal is a not to contract spec round. XM193, XM855. All not meeting a particular spec from the military contact.

https://www.pewpewtactical.com/federal-m-vs-xm-556-xm193-vs-xm855/

[mark5pt56]

With your current set up. Either you have to much gas with a bad mag or not enough gas. I meant to ask if the bolt was hitting the case head or over riding it and pushing the round partially out and then the round gets wedged as seen above. If too much gas, the bad mag won't present the round up fast enough with the bolt speed or if the bolt is coming back far enough to extract/eject but not enough to push the round from the head. Checking bolt lock multiple times will show that part. Why doesn't it happen all of the time? inconsistent hold from the shooter for one or even bad ammo. Of course a flat out bad mag alone will do this if the spring is weak or binding.

Simplest thing first is to use a good mag to check the bolt lock as stated above

[Serpico1985]

Roger that. We can do that lock back test. Seems like overgassing shouldn’t be the problem with the LMT Enhanced BCG since it bleeds off more gas than a standard BCG. Eventually my buddy will get a suppressor for it hence the LMT BCG.

I’m quite confident the ammo isn’t the problem. The gun should feed and cycle this ammo without issue.

[jbjh]

Any updates?


Sent from 80ms in the future

[Serpico1985]

Not yet. Can’t go to the range at will. Soon though hopefully

[MegademiC]

The only time Ive seen that malfunction, it was due to a bent extractor. Might want to try another bcg or bolt just to make sure there’s no issues there.

[1168]

I’ve seen similar with lightly gassed barrels and too much action spring.

Edit: look into Lysander’s advice first. He’s very knowledgeable. It also looks like there may be burring on the feed ramps. Push a cartridge in by hand with the upper off and bolt out. Push the tip up against the ramps. Does it snag?

[Serpico1985]

Thank you all for the thoughts. There is definitely a burr on the barrel ramp. You can feel it quite easily and catch a bullet nose on it. I suspect that the practice ammo was able to not get caught due to the smaller diameter nose which is why the rifle had zero issues for 300 plus rounds and many different mags. Then when he switched to the duty/HD ammo a problem surfaced in a couple different mags due to the larger diameter bullet nose.

Also, as far as gassing goes, the LMT enhanced carrier and enhanced bolt should prevent any over gassing due to the delayed unlocking and additional gas ports. Do you think the LMT part is enough to under-gas the rifle? Is the Super 42 spring known to be "stout" so to speak. I have A5's on all of my guns so I'm not current on the super 42 info.

We will test with a standard BCG soon.

But here's the question. If we determine that the problem is in fact the barrel extension ramp, how do we fix this? Seems like I could pull the barrel and very lightly sand and then cratex the ramp to eliminate the burr. I've seen School of the American Rifle on youtube do it albeit for a burr in a different location. I have a shop and quite a bit of experience with this type of work from building 1911's. But I know that AR's aren't my lane. Just curious what everyone's thoughts are if the burr is the problem.

Thanks again for your time and thoughts.

Zack

[Todd.K]

I'd put it that barrel in another upper to see if the feedramps match up better, if you can.

But if you know what cratex is you can probably smooth that out without bubba'in anything up.

[Todd.K]

And take the barrel out to work on it. If anything a slight step down from the upper is preferred.

[1168]

Thank you all for the thoughts. There is definitely a burr on the barrel ramp. You can feel it quite easily and catch a bullet nose on it. I suspect that the practice ammo was able to not get caught due to the smaller diameter nose which is why the rifle had zero issues for 300 plus rounds and many different mags. Then when he switched to the duty/HD ammo a problem surfaced in a couple different mags due to the larger diameter bullet nose.

Also, as far as gassing goes, the LMT enhanced carrier and enhanced bolt should prevent any over gassing due to the delayed unlocking and additional gas ports. Do you think the LMT part is enough to under-gas the rifle? Is the Super 42 spring known to be "stout" so to speak. I have A5's on all of my guns so I'm not current on the super 42 info.

We will test with a standard BCG soon.

But here's the question. If we determine that the problem is in fact the barrel extension ramp, how do we fix this? Seems like I could pull the barrel and very lightly sand and then cratex the ramp to eliminate the burr. I've seen School of the American Rifle on youtube do it albeit for a burr in a different location. I have a shop and quite a bit of experience with this type of work from building 1911's. But I know that AR's aren't my lane. Just curious what everyone's thoughts are if the burr is the problem.

Thanks again for your time and thoughts.

Zack

Take a picture, and mark the burrs on the photo for us, please. I’ve “fixed” a couple rifles that had burrs on the barrel extension with a tiny fine file (lightly) and some light polishing. Just want to make sure we are talking about the same thing.

[Serpico1985]

Prior to pulling the barrel I rechecked and there is a definite ledge where the barrel extension overhangs the upper M4 feed ramps. It is slight but it is there. I pulled the barrel and found that the left side ramp appears to have a chip missing out of it. Here are a couple of images to show it:

https://i.ibb.co/fHG0qgj/chip.jpg (https://ibb.co/GR3Pn6z)

https://i.ibb.co/SBFzMp6/chip-2.jpg (https://ibb.co/w45fP1d)

I have no idea how this happened. I think we would have noticed it when we did the original install. But I could be wrong. So it appears that it could be two things working together to cause feeding issues. A combination of a slightly mismatched barrel extension and upper AND a chipped barrel extension that exacerbates the situation. Thoughts?

I then put the barrel in the brand new stripped SOLGW upper and it seems to immediately be a much better fit. Here you can see that instead of overhanging there is a small gap between the bottom of the barrel extension ramp and the upper's M4 ramps. Couldn't get anything to catch on either feed ramp with this set up.

https://i.ibb.co/3FQR9SN/SOLGW-upper-1.jpg (https://ibb.co/kDvmsq8)

https://i.ibb.co/86YVH1m/SOLGW-upper-2.jpg (https://ibb.co/4P1xHhF)

I took some 300 grit sandpaper wrapped around a wooden dowel and polished the barrel extension to soften the top edge of the chip, not wanting to leave the sharp top edge there. This is really about as mundane as can be. My buddy is OCD so I removed the parkerizing or whatever the black finish is on the barrel extension on the other side too so they would match.

https://i.ibb.co/n8vPQKH/light-polish.jpg (https://ibb.co/cXHY2sG)

I called my buddy and told him what I found. I think it would clearly be irresponsible for me to do anything else to the ramp at this point. The idea of taking a significant amount off the barrel extension to make it fit the ADM upper seems foolish. We came to the conclusion that he will get a dust cover and forward assist and we will complete this SOLGW upper and test fire. Hopefully next Friday.

Question is, does anyone think this ADM upper will work with other barrels? I'm not sure if they would replace it or agree that the ramps are a little shallow, which they seem to be based on the SOLGW upper comparison. Maybe if they won't replace it my buddy could get store credit or something.

Let me know what y'all think. I appreciate the comments and assistance.

[Alpine2k3]

I would check the bolt to make sure it’s not damaged. That barrel extension looks awfully rough.


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[Serpico1985]

No bolt damage, may just be the lighting giving that impression.

UPDATE 10/2/2020

Got to the range today and was able to test the rifle in the new upper. Remember I swapped the ADM upper for a SOLGW upper. Results are as follows. Keep in mind last time the gun was failing to feed every other round with pmags and 20round colt mags

-With the new upper (SOLGW) we made it through about 10 rounds of the duty ammo from the pmag before we got the same malfunction as last time:
https://i.ibb.co/fd7Xbsx/IMG-7846.jpg (https://ibb.co/N2zFXMy)

-So we swapped the Geissele super 42 buffer and spring for a standard carbine buffer and spring, topped off the mag with more duty ammo and blazed through the entire mag with zero issues.

-Then we put the Geissele super 42 buffer and spring back in and replaced the LMT enhanced BCG and enhanced bolt with a standard mil-spec BCG and bolt. Loaded back up with duty ammo and shot the mags empty, no problems.

-Finally we shot his upper with his LMT enhanced BCG and enhanced bolt on a SBR lower with an A5-2 buffer and blazzed through a couple mags of duty ammo without issue.

My best guess is that the combination of the super 42 buffer and spring (noticeably stouter than the standard carbine spring and A5 spring) when combined with the LMT enhanced BCG and enhanced bolt (which reduces gas pressure vs a standard BCG) was not giving enough gas to fully run the gun. Which is why replacing one or the other with a standard unit let the gun run. Also, best guess for the barrel extension overhanging the ADM upper's feed ramps exacerbated the original feeding issue which is why it was failing every other round vs going 10 rounds today without issue.

Am I drawing conclusions to fit my assumptions? Probably. But whatever, the gun is running now with all his ammo in all his mags so we'll call it gtg.

Thanks

[everready73]

That makes perfect sense to me

[MC_Oper8or]

The super 42 kit seems best in over gassed guns.


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