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

**Serpico1985** · 09-19-20, 13:59

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:

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:

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:

The dents match up to the lugs on the bolt.

On the opposite side of the dents there is a deep gouge/scratch:

He described the orientation of the round in the upper when the failures occurred like this:

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:

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** · 09-19-20, 14:10

The exposed lead nose is snubbing on the feed ramp.

**Disciple** · 09-19-20, 14:44

Originally Posted by lysander

The exposed lead nose is snubbing on the feed ramp.

What can be done about that?

**Serpico1985** · 09-19-20, 15:08

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** · 09-19-20, 15:30

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** · 09-19-20, 15:45

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** · 09-19-20, 16:06

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

**Serpico1985** · 09-19-20, 18:25

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** · 09-19-20, 18:49

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/feder...m193-vs-xm855/

**mark5pt56** · 09-19-20, 19:09

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