View Full Version : Interesting failure
I have seen lots of different failures of parts over the years, but this is a first for me in 20 plus years of military M-16/M4 use and another 15 of civilian AR-15 use.
Yesterday, I was exercising a clean and properly lubed mid-length 16" 5.56 chambered (verified) carbine mounted with Luepold VX-R Patrol optics with a Spike's Tactical lower with their Nickel-Boron single stage trigger. I had just swapped out the buffer to a slightly heavier Spikes T2 (it had an unmarked CAR buffer previously) as I was having a very sporadic issue with the bolt not locking back on the last round with 5.56 rounds (not .223) prior to the swap. The gun (except for the buffer) had close to 1000 rounds through it, with 500 being Federal AE .223 and the balance 5.56 XM193.
I had a mixed stack of twenty-two 20 round mags by C-products and Brownells, each loaded with 1 round of 5.56 XM193. After the 3d shot (bolt locking back each time) and with a new mag in and bolt released, I squeezed the trigger and felt it wasn't reset properly. When I released the pressure it reset and I sqeezed again and it functioned. After the "what now" thought of a problem with the buffer, quickly dispelled as the bolt had locked back previously, I thought of fouling in the trigger mechanism of some type. I cleared the weapon, broke it open on the line and moved back to a bench with the lower to look at it with my tools/gear back on the bench.
I found that the Hammer pin had sheared in half at the inner groove and only the left half of the pin was still in place in the gun. I replaced the pin with a spare from a repair kit I keep in my range bag.
Casual visual inspection of the pin showed no flaws other than it was a clean break. By happenstance, a group of 20-30 Federal Marshals and IRS agents were doing quarterly training and a couple of the Marshals with whom I had casual acquaintance came over to look at what I was doing. They had one of their fellows, a trained armorer, come over to take a look at the pin. While I had never seen that type of failure, he told me that they had a single M4 Colt that has had 3 hammer pin failures (!!!!!!!!!) like that, but he had seen no others of that type.
Out of concern, I fired another 240 rounds, 44 single round per mag and the balance full mags, out of it after replacing the pin with no further malfunctions of any kind and the bolt locked back each time it was supposed to.
What did I learn (or reinforce that I already knew)......well, it pays to always carry repair parts of all types with you. It also pays to carry extra rounds so that if you have a failure and repair on site, you can exercise the gun right then and there to see if it functions and is reliable afterwards.
Other than that, failures happen, but it was the first hammer pin failure I have seen.
I believe that other members have posted pics of some failures as well I have seen one or two myself. My guess is that the pins had an improper heat treat.
I suspect so also, as the break was clean and the structure appeared to be crystaline. Too brittle is my guess, but I have nothing to back that up other then having seen brittle strike failures in other metals in other types of equipment. It looked not unlike what you would see in cast iron from a strike break.
It was the pin provided by Spikes with the trigger kit. It has been replaced with a pin from a repair kit from Brownells.
Time and round count will tell. I see that it isn't uncommon......just uncommon to me in particular. Wish I had thought to ask where the Marshall got his pins from as it which might explain why one gun had three failures. They may have been from all of the same batch of improperly treated pins.
Thanks Gunz
I suspect so also, as the break was clean and the structure appeared to be crystaline.
Thanks Gunz
Metal is a crystalline substance, you need a microscope to check out grain structure...
+1 on the spare parts though.:)
Metal is a crystalline substance, you need a microscope to check out grain structure...
+1 on the spare parts though.:)
Yes, I know, but bend (fatigue) failure has a different look as does tear (stretch) failure from strike (fracture) failure. But as I said, it was a casual visual inspection with a naked eye and surely my old eyes even with good glasses aren't that good!
How's the top of your disconnector look? I've seen this caused by overgassed/underbuffered rifles where the hammer was being forced back harder by a fast moving bolt group.
I've also seen it caused by a rifle firing a lot of rounds suppressed.
How's the top of your disconnector look? I've seen this caused by overgassed/underbuffered rifles where the hammer was being forced back harder by a fast moving bolt group.
I've also seen it caused by a rifle firing a lot of rounds suppressed.
Thanks for the insight. No suppressor use on this one. The disconnector looks fine, but it was having problems with the bolt not locking back from having too light of a buffer. I was exercising the gun after recently changing buffers. I went from an unmarked CAR buffer to Spikes T-2 (4.1oz), so the previously light buffer could have been a contributing issue. But it is a mid-length 16" so it is milder in the first place. (Wilson fluted chrome-moly 5.56 chambered barrel with upper assembled by LMT using an AR-15 BCG). But what you are saying makes perfect sense.........The first 500 rounds through it were milder Federal AE223 with no issues of any kind. The next close to 500 rounds were milspec LC 5.56 XM193 and that is where the failure to lock back issues began to manifest from having too light of a buffer. Improperly heat treated pin (that is just a guess, but has been proposed by IG previously) added to a light buffer and heavier, harder bolt rearward movement from milspec ammo= premature pin failure.
Since the original post, I have given everything a detail cleaning and inspection and everything appears normal (sure you can't see internal structure issues, but no outward indicators were evident).
I haven't had an opportunity to shoot it since the detail cleaning, but I will be out in the field tomorrow and will stop by a whistle pig area and shoot a few hundred rounds and see how it goes.
It has had a little over 200 rounds thru it after replacing the buffer and the pin with no issues, but I would like a few hundred more before I am comfortable.
A pin broke.
Yes, a hammer pin. The reason for the OP was that it was an odd failure to me in my years of use the platform. Others have varied experiences.
May not seem big, but if you can't pull the trigger and make the gun work, it doesn't matter how small or insignificantly seeming the part is.
Thanks for the insight. No suppressor use on this one. The disconnector looks fine, but it was having problems with the bolt not locking back from having too light of a buffer. I was exercising the gun after recently changing buffers. I went from an unmarked CAR buffer to Spikes T-2 (4.1oz), so the previously light buffer could have been a contributing issue.
this goes against common knowledge. generally, if a bolt doesn't lock back, you go to a lighter buffer. maybe you had this problem before you changed buffers...
this goes against common knowledge. generally, if a bolt doesn't lock back, you go to a lighter buffer. maybe you had this problem before you changed buffers...
A buffer that's too light on an overgassed gun will cause the bolt to outrun the mag follower, and thus no lock back.
A buffer that's too light on an overgassed gun will cause the bolt to outrun the mag follower, and thus no lock back.
I understand. but how often is that the norm? generally with way too big gasports or with suppressor, no???
magic 8-ball says probably had broken hammer pin before buffer change...
https://www.m4carbine.net/showthread.php?t=124949
That's my thread... but it was not the hammer pin - it was the trigger pin.
http://img19.imageshack.us/img19/9457/brokenpin4.jpg
http://img5.imageshack.us/img5/33/brokenpin3.jpg
Did it look like that ^^^^^ ?
^^^^^^^^^^^^^
Exactly like that...same look to the break and all.
Too light of a buffer can prevent lock back of the bolt. I know it may seem counter intuitive to you, but if you read the buffer threads and posts on like malfuntions, a heavier buffer solves the issue nicely.
And I believe your magic 8 ball is wrong, RA2Bach. Prior to looking for a change of buffer, a complete detail and cleaning inspection was made to preclude any other issues before screwing with things. With that said, I bet you're right it was a significant causal factor though.
The consensus on my thread was that it was a crappy pin.
Luckily it was very good CS on part of who I purchased the lower from and I had a new pin (multiple) in a few days.
wetidlerjr
03-27-13, 14:42
https://www.m4carbine.net/showthread.php?t=124949That's my thread... but it was not the hammer pin - it was the trigger pin.
http://img19.imageshack.us/img19/9457/brokenpin4.jpg
http://img5.imageshack.us/img5/33/brokenpin3.jpg
Did it look like that ^^^^^ ?
Who knows? A smaller pic that wasn't fuzzy might help.
The shop where I used to work had a Colt 635 on the rental wall and that joker used to break quite a few hammer pins. Is that a known issue with 9mm AR's due to the blow-back vs gas operation?
^^^^^^^^^^^^^
Exactly like that...same look to the break and all.
Too light of a buffer can prevent lock back of the bolt. I know it may seem counter intuitive to you, but if you read the buffer threads and posts on like malfuntions, a heavier buffer solves the issue nicely.
And I believe your magic 8 ball is wrong, RA2Bach. Prior to looking for a change of buffer, a complete detail and cleaning inspection was made to preclude any other issues before screwing with things. With that said, I bet you're right it was a significant causal factor though.
damn magic 8-balls, they don't make'em like they used to ...:rolleyes:
but anyway, I stand by my original statement - MOST times a bolt not locking back is caused by being undergassed, low power ammo, or too heavy a buffer...
Yes, that is very common with the 9mm uppers. That is why Colt came out with the stainless pins.
I've gone through quite a few on my select fire lower until I installed the KNS pins. Haven't broke one since.
The shop where I used to work had a Colt 635 on the rental wall and that joker used to break quite a few hammer pins. Is that a known issue with 9mm AR's due to the blow-back vs gas operation?
Who knows? A smaller pic that wasn't fuzzy might help.
Figuring the OP could see it fine and the pictures are in an acceptable resolution. I'm going to assume that it's your computer / phone. You're more then welcome to click on the link I provided and look at the pictures there.
wetidlerjr
03-28-13, 12:01
Figuring the OP could see it fine and the pictures are in an acceptable resolution. I'm going to assume that it's your computer / phone. You're more then welcome to click on the link I provided and look at the pictures there.
Your assumption is incorrect as I have a 26" hi-res monitor set at a 1360x768 and it took up all of it with the need to scroll about 50% farther to see the other side. I know how to see a smaller version simply by opening only the pic in another tab but it's a whole lot easier for everyone to just post a smaller pic. If not then carry on.
Your assumption is incorrect as I have a 26" hi-res monitor set at a 1360x768 and it took up all of it with the need to scroll about 50% farther to see the other side. I know how to see a smaller version simply by opening only the pic in another tab but it's a whole lot easier for everyone to just post a smaller pic. If not then carry on.
I (the OP) see the picture and the size and resolution are just fine. Nobody else has complained but you.
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