Help with failures to extract ... found potential cause ... do not understand

[youreacrab]

After firing 30 rounds of GGG m193 with no stoppages, I switched to 75 gr Hornady factory match ammo. Fired 40 rounds Hornady with no stoppages, then on the last box of 20, I started having repeated failures to extract.

Each time, I found the fired case stuck in chamber with next round jammed with its nose in the chambered case's primer. Cleared by dropping mag, manually cycling bolt to drop the extractor on the chambered case, then manually cycling bolt to clear the chambered round. This worked every time except for the last time, when the extractor wouldn't hook onto the spent case. Other interesting note is that some of the last rounds of Hornady caused a small cloud of gray smoke to puff out of the seam between the upper and lower.

Gave up after 5 failures, came home, pushed the chambered case out with a cleaning rod (came out easily), and disassembled the bolt. Out of the bolt and carrier dropped these tiny pepper-like shards that I assume came from the ammo. I found one lodged between the extractor and the bolt, which explains why the last round couldn't be grabbed.



Barrel is Noveske CHF chambered in 556 NATO. Less than 500 rounds total and reasonably clean.

Can anyone help me make sense of this?

Thanks

[Clint]

Those are bits from pierced primers.

Check your spent brass and you should find extra holes that don't belong.

(for completeness) Pierced primers are caused by a combination of 4 factors:

1) loose fit between firing pin and bolt.

2) thin / weak / soft primers

3) excessive chamber pressure from hot loads or tight chamber

4) overgassing / early extraction


In this case, it is most likely cause an out of spec bolt with a wallowed out firing pin hole.

The spec for the firing pin hole is .0635 +/- .001.

[twm134]

For what it's worth I have experienced pierced primers when using Hornady 5.56 75gr ammo in a BCM 16" middy (BCM bolt) that had never experienced pierced primers before in over 1800 rounds of various hand loads and factory loads. Factory loads included Black Hills 5.56 77gr, a couple brands of m193 and m855, as well as several 223 factory loads.

I only had a 20 round box of the Hornady 75s and haven't experienced it with anything else since so I didn't follow up on the issue. I just don't use Hornady 5.56 75 loads.

[youreacrab]

Thanks Clint and twm134.

I picked up some brass, and did notice some deep gouges in the primers of the hornady (including one hole). I figured it was from the OTM bullet of the jammed round taking a chunk out when it jammed.

Based on the picture, how many pierced primers do you think that is? I'm not sure whether it was the Hornady or the GGG that was pierced...didn't notice anything wrong until rounds 41-45 of the Hornady failed to extract.

In the pic below, three Hornady's have a large primer crater and were rammed with the OTM of the following jammed round. Two Hornady's were shot with only one round in the mag (and do not have the large crater). And two GGG as well.
Wish I would have picked up more of my brass from the range floor...

[TomMcC]

You're definitely getting primer flow around your firing pin. Did you check to make sure the firing pin to bolt fit is good? It's probably going to be hard or impossible to measure without the proper tool. There should be almost zero play between the firing pin tip and the bolt when protruding. Did you check the other brass for flow or piercing? If no flow or piercing, I'd say the ammo is too hot or the primers are too soft.

[youreacrab]

Thanks all. I've decided I'm going to ship the gun back to the manufacturer. Doubt its an ammo problem if its happening on the m193 and the Hornady 233.

[Clint]

That is a great example of the combination of factors that was listed before.

Notice the mil spec primer in the Nato GGG is flowing badly, but not piercing.

The Hornady with no doubt commercial primers are piercing more often than not in this small sample.

The ones that didn't pierce were single fired, probably in a cooler chamber and with slightly lower pressure.

[Locutus]

I have had the same problem with Hornady's .303 Match ammo. It was blowing so many primer particles into the bolt head that it jammed the firing pin and I had to take the rifle home to disassemble with tools - it was not pretty. I switched to Prvi Partizan ammo and the problem went away. I've been loading Hornady bullets for decades and been quite happy with them, but I'll never shoot their loaded ammo again.