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AR-10B Hammer Issue

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Well I took her out to the range again this past weekend for a much overdo range trip but less than a mag into it, 15 rounds to be exact, she suffered a catastrophic failure.  Problems seemed to occur in the previous couple of rounds as it started to have ejection issues.  The previous round would be stove-piped out the side with the next round almost in the chamber.  This was annoying and it was something that had NEVER happened before.  In fact the AR-10 had previously experienced zero functional failures. 


Then when I went to fire round 16, nothing.  Not even a click.  Clear the round and chamber another... same thing not even a click.  Remove the mag and clear the firearm and try to dry fire... nadda... nothing..   So I open it up to find the hammer broken in half at the weakest point.  Now this rifle has less than 500 rounds through it so I seriously didn't expect this.  It clearly looks like it broke because of a defect in the metal.  It would seem to be a molded or cast part with an air bubble at probably the worst place or it to happen. 


Now I haven't called Armalite yet and I wouldn't be surprised if they send me a replacement part for free.  However in looking at the stock part vs my AR-15 the hammer is not only molded it's a lot thinner and lighter than the one in the AR-15.  So I am wondering if anyone knows if there is an aftermarket hammer that is a direct swap out part that would work in an AR-10.


For those interested this is what I found when I opened the rifle up:







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1.  It would seem to be a molded or cast part with an air bubble at probably the worst place or it to happen. 


2.  Now I haven't called Armalite yet and I wouldn't be surprised if they send me a replacement part for free. 



1.  MIM.  Metal Injection Molding.  It's becoming more common in parts these days.  For the most part, it's not a bad thing, despite the schit-talking that goes on about it. The firearms world is not the only place it's used.


2.  If that was a Gen-U-Wine Armalite lower parts kit, they'll replace that hammer for you.  Get ahold of them.

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Yup I do plan to call Armalite.  It's just been crazy this week work-wise being the end of year so that had to come first.  So I will later this week or early next.  What I was really wondering is if there was something short of a complete Timney unit that I could replace it with a non MIM part.


As for MIM I am aware of the issues people have against it.  I have plenty of things with a lot of rounds through them that have MIM parts though.  However this issue does point out the potential issue with them.  I think a bubble inside a hammer forged piece of steel is a lot less likely.  That being said in general I have never had an issue with them until now. 


It is the original part that came in the rifle new.  So I am sure it's factory Armalite.  BTW this is no slam on Armalite either.  Every Mfr. has some small amount of bad parts get through.  I have had no issues with any of my Rugers or my Kimber both of which take a beating in some circles for the use of MIM parts.  Others however have.  Nobody is perfect. 


I was just wondering if there was a direct swap out forged part that was maybe heavier than the stock.  For example if I bought an AR-15 one would it work in this rifle?

Edited by jfeenin
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Or, if that's just a standard trigger and you want to replace it with a good standard trigger...


The Geissele-manufactured ALG Defense ACT trigger.  That one won't break.

Looks like a two-stage to me. You can tell by the dual catches on the head of the hammer (piece that broke off).

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Armalite makes a 2-stage (nice) trigger, then the 2-stage National Match trigger - which is a damn nice trigger.  I know I'm a Geissele whore, but I've got one of the Armalite NM triggers, and I refuse to replace it with something else.  It's migrated through a couple rifles, stayed in the Mk12 for awhile, but it's new permanent home is in the 300BLK rifle. 

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  • 2 months later...

Yes, I know, old topic but I just thought I would post an update.  I did call Armalite back in January but I think I got the wrong extension and someones voicemail saying they were out of the office until the 22nd.  I was pretty busy and decided to call back later and well, life happens, I got sidetracked and it became one fo those cases of when I remembered i had to I couldn't and when I did have the time I never thought of it.  Until last week when I called and used the extension listed on their website for RMA's etc.  I got right through and a very friendly lady happily took my name and address and said she would get a new hammer right out to me.  It arrived today...  Not THAT is service.   Two thumbs up for Armalite there.  Now I just have to put it in.


I do note the mold is different than it was with mine and I can't help but wonder if what happened to me might be part of why.  There are notably indents in the side of the new one that weren't in the original.  Who knows?  All I know is that with any luck in a few hours I will have a running AR-10 again.   :)





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Well changing the part took maybe two minutes with my Glock tool to push out the pin and little needle nose to handle the D-ring without flopping it into the carpet with my butterfingers.  

Here you can see the differences in the mold used pretty clearly.

Since I had the real camera out for once I decided to use some extension tubes and see if I could get some real decent close-up shots of the break.  This is the results, pretty cool looking if nothing else:



What is clear in these photos that wasn't in my not so great phone pics is that the hole clearly went up into the top of the hammer as well.  

With any luck I will have time to put a few rounds through it this weekend to test it out.

Edited by jfeenin
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Could be.  All I know is that it was a serious bummer when it broke but Armalite replaced it with no questions asked which was good.  Hopefully the new one will not do that.  If it does I may just go with a single stage trigger which looks to have a beefier hammer overall.  That hammer in the linked post had the same thin area in the design but the top part of the hammer appears to be a different shape than the Armalite one.

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