Colt 6933 chokes on PMC bronze

[jpaul]

I recently acquired a used Colt 6933 upper to use w/ a 6921 lower. I took it to the range to sight in a KAC flip up rear sight, so I grabbed 2 boxes of PMC bronze to take w/ me. I wasn't expecting any issues, so I was pretty surprised to have multiple cases fail to eject. I've never used this ammo before, but it seemed noticably weaker than the federal xm193 & xm885 I usually have. The only "upgrades" installed are a Colt type I rounded hammer, a green extractor spring from SAW, & a Surefire flash hider / adaptor for my 556-212 can.

The malfunction left the fired case in the ejection port. The next live round couldn't enter the chamber because of the empty case,so it was wedged between the bolt and the barrel extension. This didn't occur w/ every shot, but probably once every 3 rounds or so. I don't think it was "short stroking" as the bolt attempted to chamber the next live round & it also locked back after the last round was fired each time.

For the sake of experimentation, I attached the suppressor, & didn't have any more issues. I've never really placed much emphasis on ejection pattern before, but with the suppressor, the cases landed around 2:00. Without the can, they were probably bordering 5:00. Before anyone asks, a generous amount of Slip 2000 EWL was applied before hand.

My expedient resolution is to not use the PMC bronze in this upper. W/ that said, what does anyone else think? Ammo problem or something else? Next time out, I may try a dfferent bolt. I'll also test this ammo in my other 6933 to see if I have the same problem. Any thoughts or suggestions are welcome.

[MistWolf]

You have an extraction issue. Replace the extractor spring. Use a Colt spring

[Arik]

I recently acquired a used Colt 6933 upper to use w/ a 6921 lower. I took it to the range to sight in a KAC flip up rear sight, so I grabbed 2 boxes of PMC bronze to take w/ me. I wasn't expecting any issues, so I was pretty surprised to have multiple cases fail to eject. I've never used this ammo before, but it seemed noticably weaker than the federal xm193 & xm885 I usually have. The only "upgrades" installed are a Colt type I rounded hammer, a green extractor spring from SAW, & a Surefire flash hider / adaptor for my 556-212 can.

The malfunction left the fired case in the ejection port. The next live round couldn't enter the chamber because of the empty case,so it was wedged between the bolt and the barrel extension. This didn't occur w/ every shot, but probably once every 3 rounds or so. I don't think it was "short stroking" as the bolt attempted to chamber the next live round & it also locked back after the last round was fired each time.

For the sake of experimentation, I attached the suppressor, & didn't have any more issues. I've never really placed much emphasis on ejection pattern before, but with the suppressor, the cases landed around 2:00. Without the can, they were probably bordering 5:00. Before anyone asks, a generous amount of Slip 2000 EWL was applied before hand.

My expedient resolution is to not use the PMC bronze in this upper. W/ that said, what does anyone else think? Ammo problem or something else? Next time out, I may try a dfferent bolt. I'll also test this ammo in my other 6933 to see if I have the same problem. Any thoughts or suggestions are welcome.

I had the same issue with Independence ammo in blue and white box. The rifle was a bone stock 6920. Had it out in a class the week before. It was rain and cold, I was using Wolf Gold without problems. Took the rifle out two weeks later with a few boxes of Independence and had those same issues. It was the only ammo to give me that problem so I just never bought anymore. And just in case got rid of that mag.

There a topic here somewhere about it

https://www.m4carbine.net/showthread.php?t=190374

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[jpaul]

You have an extraction issue. Replace the extractor spring. Use a Colt spring

That's the plan for later today....try w/ a Colt extractor spring.

[jpaul]

Thanks for the link. The last picture in your thread looks very similar.

[Caduceus]

You have an extraction issue. Replace the extractor spring. Use a Colt spring

Probably this, but Bronze is a little weaker than m193. Probably not enough to be tjat problematic.
I tend to use bronze to get me on paper with a new build, then m193 for final sight in.

[mig1nc]

Just wondering, why do we think it's an extractor issue if it works suppressed 100% with PMC and when using normal pressure ammo unsuppressed? PMC bronze gives me issues too from time to time. It's not just week as shit, it's also inconsistent it seems.

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[MistWolf]

I'll try to explain why it's an extractor issue, step by step.

1) The ejector is a spring loaded plunger. It presses the case outward until the front of the case clears the front of the ejection port. Therefore, as the case is being extracted, it's making contact with the rifle at three points. The ejector plunger, the extractor and the right side of the receiver. These three points hold the case in place as it's being dragged out of the chamber until it clears the front portion of the ejection port and is flung clear of the action by the ejector.

2) A fresh round cannot be stripped from the magazine until the bolt face travels far enough to the rear that it clears the rim of the case. The bolt will not push a case from the magazine by pushing on the body of the case. It MUST catch the rim. You can test this for yourself by loading a magazine and working the charging handle. Pull the charging back to any point before the bolt would catch the rim and let it fly forward. It won't push the round out of the chamber. Here, I will insert the standard issue safety warning about testing the action with live ammo. Make up a few dummy rounds made from brass cases to perform this test.

3) An empty case will eject long before the bolt travels far enough to pick up the next round. Therefore, it's possible for an AR to short stroke and eject a round at a point where the bolt will not strip a fresh round from the magazine. This can be tested by chambering an empty, loading a magazine with dummy rounds, then pulling back the charging handle just to the point where the empty is ejected. Once the empty is ejected, let the charging handle fly forward. The dummy round will not be pushed from the magazine. NOTE: Inserting an empty case in the chamber will result in the case getting stuck in the chamber. If you decide to perform this test, only use an unfired or a properly resized case.

4) The case must be held against the ejector plunger in order for the ejector plunger to fling the case from the action. In a properly functioning rifle, the case is held by the extractor claw. The extractor claw is held in place by spring pressure. If the spring pressure is weak, the claw can slip over the rim and lose control of the case during extraction.

5) The magazine can do nothing that causes the the case to be knocked loose from the extractor during extraction. The only way the magazine could cause problems with extraction/ejection is if the top round were to knock the extracting case loose from the extractor before ejection can occur. However, the top round is held down by the bolt & carrier until the bolt clears the rim. Long before the bolt clears the rim of the top round, the empty case will be ejected. Therefore, the magazine cannot cause extraction/ejection malfunctions.

6) The malfunction we are discussing is not a double feed. A double feed is when two live rounds are fed from the magazine at the same time. A double feed is always a magazine issue. However, what we are seeing here is a live round and an empty being jammed in the action. Therefore, the cause is due to an empty case being left in the cation.

Let's put all of this together and see what's happening. The empty isn't being ejected because the the extractor loses control of the case before the case can be ejected by the ejector.

The case, having been released prematurely by the extractor, now lies in the action. The carrier continues to the rear, enough that the bolt clears the rim of the top round in the magazine. On its return, the bolt catches the rim of the top round to strip it from the magazine, but it is blocked before it can be fed into the chamber.

If the empty case failed to eject due to short stroking, the carrier would not have traveled rearward enough for the mouth of the case to clear the ejection port and therefore would not be able to strip the top round from the magazine. If short stroking resulted in ejection without enough rearward travel of the carrier for the bolt to clear the rim of the top round, the carrier group would simply close on an empty chamber, or simply not go into battery on an empty chamber. We can eliminate short stroking as the cause of this malfunction.

If the rifle were suffering from weak ejection, the ejector would push the case outward and the top round would normally push the empty out of the way. This could result in the top round being partially stripped from the magazine as the empty can impede the feeding of the fresh round as the empty is being knocked clear of the ejection port by the top round. It can also result in last round stove pipes. We can almost certainly eliminate weak ejection as the cause of this malfunction.

Pressure in the bore helps press the case against the ejector plunger during extraction. As a suppressor keeps the pressure in the bore from dropping as quick, the pressure in the bore during extraction will be higher. This will improve extraction, but only serves to hide the fact that the extractor spring is weak and needs to be replaced. Once a spring begins failing, it continues to fail with each cycle and it's just a matter of time before it fails completely. The frequency of leaving extracted cases in the action will continue and soon the extractor will start leaving empties in the chamber.

Over gassing will not cause a good extractor and a good extractor spring to lose control of the fired case. What it will do is overload a spring that has started to fail. A good extractor and spring will tear the rim off a stuck case before it will jump the rim and let go of it. While over gassing will expose the problem, over gassing isn't the cause.

I hope this explains why this malfunction is an extraction issue, not short stroking, a magazine issue or over gassing

[P2000]

^^^ good post

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[jpaul]

I had a brief break in the middle of the day, so I took the SBR out w/ 60 rounds of the same ammo. Everything was the same except the extractor spring. I replaced the green SAW spring w/ a Colt factory extractor spring. I proceeded to fire 40 trouble free rounds. At this point, I reinstalled the green extractor spring thinking the malfunction would occur again thus proving definitively, that was the root cause. Unfortuneately (or fortuneately?) I made it through the last 20 rounds without an issue.

60 trouble free rounds isn't quite enough to convince me of anything yet. Just reporting my findings. I'll likely test it more thoroughly later. Only trouble is, I don't have much more of the PMC bronze left (assuming that was either wholly or partially respnsible). Luckily for me, this carbine is strictly for fun.