My 9mm AR went full auto at the range today. Twice. - Page 1

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[ARCHIVED THREAD] - My 9mm AR went full auto at the range today. Twice.

Posted: 5/12/2017 1:28:37 PM EDT

First a three round burst, then a two round burst half of a mag later.

QC 10 lower
Nickle-plated mil spec FCG (I think from Right to Bear)
Kaw Valley BCG
Kaw Valley 9mm buffer
Standard carbine buffer spring
Faxon 8.5" barrel
33-round Glock factory mag
Brown Bear 115 gr 9mm

The same rig went through 60 trouble-free rounds of the same Brown Bear ammo fed by two different 31 round ETS mags just prior to this occurrence, and the bolt locked open on both mags.

I packed it up after that.

?

Posted: 5/12/2017 1:42:28 PM EDT

[#1]

There's a black helicopter over your house as we speak OP. Dibs on whatever the JBT's leave after the raid. And board your dog at a friends house to ensure your pet's safety

In all seriousness though, I don't know how that happened. Tagged for some knowledge.

Posted: 5/12/2017 1:47:40 PM EDT

[#2]

Firing pin got stuck in the face of the bolt.

Posted: 5/12/2017 1:49:28 PM EDT

[#3]

Quote History

Quoted:
Firing pin got stuck in the face of the bolt.

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So it was slamfiring?

Posted: 5/12/2017 1:51:12 PM EDT

[#4]

A broken hammer pin will do that. It happened to me. Doubles triples, and then almost an entiremag............then dead. Sheared off.

KNS pins are a good thing. So is a ramped bolt.

The other time I had this happen was I fooked up and installed a small pin FCG in my large pin Colt. Doubles and triples.

Posted: 5/12/2017 2:31:05 PM EDT

[#5]

Quote History

Quoted:
A broken hammer pin will do that. It happened to me. Doubles triples, and then almost an entiremag............then dead. Sheared off.

KNS pins are a good thing. So is a ramped bolt.

The other time I had this happen was I fooked up and installed a small pin FCG in my large pin Colt. Doubles and triples.

View Quote

I forgot to mention that this lower has KNS anti-rotation pins.

Posted: 5/12/2017 3:08:29 PM EDT

[#6]

New build?

Does it pass function check?

Probably FCG, or you inadvertently 'bump fired' it (RTBA trigger has lighter springs than mil-spec).

Posted: 5/12/2017 3:31:31 PM EDT

[#7]

Quote History

Quoted:
New build?

Does it pass function check?

Probably FCG, or you inadvertently 'bump fired' it (RTBA trigger has lighter springs than mil-spec).

View Quote

This is its third range outing. I've probably put ~400 rounds through it during the last two trips. The only issues I've had with it thus far is failure to lock back on an empty mag, which I've been tinkering with. It seems fine just running the action and doing a function check.

I've never even tried purposefully bump firing, but I can give it a whirl next time I hit up the range.

Posted: 5/12/2017 3:56:43 PM EDT

[#8]

Yea, I would think either the hammer/disconnector is out of spec, or the firing pin is stuck forward somehow. Either way it is slam firing as the bolt returns forward into battery. If it is the firing pin stuck in the channel, then that would make it into an open bolt action heh.

Posted: 5/12/2017 4:25:39 PM EDT

[#9]

Quote History

Quoted:

I forgot to mention that this lower has KNS anti-rotation pins.

View Quote

What else did you fail to mention?

Posted: 5/12/2017 5:24:26 PM EDT

[#10]

A friend had the exact thing happen on a 9mm AR build. Could not find a single problem in the FCG, it was just throwing random doubles and triples.

Believe it or not, a new buffer spring cured it.

Posted: 5/12/2017 6:38:26 PM EDT

[#11]

my guess, perhaps bolt not coming back far enough to reset hammer on disconnector as part of the equation? to compound what was already mentioned above. so a less-stiff buffer spring would allow more reliable full bolt travel? just guessing here.

Posted: 5/12/2017 10:37:02 PM EDT

[#12]

Disconnect finger not have deep enough grab on hammer shelf.. CLUNK of bolt hitting home shake it enough to knock disconnect finger off hammer and let it fly

Posted: 5/13/2017 12:05:43 AM EDT

[#13]

It happened to me recently, swapped out FCG with a spare to start troubleshooting (no pun intended), and so far so good the last hundred or so rounds.

Posted: 5/14/2017 7:41:17 PM EDT

[#14]

Quote History

Quoted:
It happened to me recently, swapped out FCG with a spare to start troubleshooting (no pun intended), and so far so good the last hundred or so rounds.

View Quote

That cured my issue as well. Now I use an ALG ACT with no problems.

Posted: 5/15/2017 3:34:02 PM EDT

[#15]

I'm having a similar issue with a drop in trigger. Switched to a mil spec to run tomorrow to try and pinpoint where the issue may lie.

Posted: 5/16/2017 8:13:41 AM EDT

[#16]

Look at your disconnector. You will see a spot where the tail of the hammer is hitting it. You will need to cut the tail of the hammer completely off or use a 9mm hammer.

Posted: 5/16/2017 7:20:40 PM EDT

[#17]

Quote History

Quoted:
Look at your disconnector. You will see a spot where the tail of the hammer is hitting it. You will need to cut the tail of the hammer completely off or use a 9mm hammer.

View Quote

Before -

Attached File

After -

Attached File

Hitting trigger & disconnect,, suspect hammer bind is what cause hammer pin breakage for some

Attached File

I have been 9mm since mid 2003, have not broken any pins here, but did cut tail off neutered M16 hammer about a year ago

Posted: 5/16/2017 7:25:07 PM EDT

[#18]

Nevermind, smarter people said smarter stuff

Posted: 5/16/2017 7:38:45 PM EDT

[#19]

Quote History

Quoted:

Before -
https://www.AR15.Com/media/mediaFiles/997/ar-223-209894.JPG

After -
https://www.AR15.Com/media/mediaFiles/997/AR-15-hammer-bobbed-209892.JPG

Hitting trigger & disconnect,, suspect hammer bind is what cause hammer pin breakage for some
https://www.AR15.Com/media/mediaFiles/997/ar-15-inside-209893.JPG

I have been 9mm since mid 2003, have not broken any pins here, but did cut tail off neutered M16 hammer about a year ago

View Quote

Why is the hammer hitting the disconnector like that?

Posted: 5/19/2017 10:51:29 AM EDT

[#20]

Quote History

Quoted:
Why is the hammer hitting the disconnector like that?

View Quote

My guess is that the violent impulse of the blowback operation slams the tail of the hammer into the disconnector.

The impact on the disconnector may or may not prevent it from catching the hammer which would lead to doubling (or more) due to hammer follow.

Posted: 5/19/2017 11:38:12 AM EDT

[#21]

It's pretty easy to bump fire even from the shoulder when recoil is high. Our lighter trigger may have helped with that.

If you feel like it may be the disconnector shoot me an email and we'll get your trigger group swapped out.

[email protected]

Posted: 5/19/2017 12:19:14 PM EDT

[#22]

Mine went full auto on an occasion...........I was using a sprinco red spring and one of those long/heavy stainless buffers..........I was firing bursts and it ran full auto until it fired out of battery and blew the extractor off the bolt..........

With a standard spring it didn't go full auto.......but once I installed the Sprinco Red spring it did................I replaced the heavy buffer with a RRA 9mm buffer and installed a rubber bumper to restrict over travel of the bolt...No full auto issues since. I inspected the FCG and found no issues ......

Posted: 5/22/2017 6:42:31 AM EDT

[#23]

Obviously, you need to install a second set of KNS pins, 'cause the first set didn't fix it enough.

Posted: 5/23/2017 8:41:30 PM EDT

[#24]

Could this be caused by a missing or broken firing pin spring?

Posted: 5/23/2017 9:00:28 PM EDT

[#25]

I had that issue with a non ramped bolt and a standard hammer. I've switched to ramped bolts now.

Posted: 5/30/2017 1:04:48 AM EDT

[#26]

Quote History

Quoted:
Before -
https://www.AR15.Com/media/mediaFiles/997/ar-223-209894.JPG

After -
https://www.AR15.Com/media/mediaFiles/997/AR-15-hammer-bobbed-209892.JPG

Hitting trigger & disconnect,, suspect hammer bind is what cause hammer pin breakage for some
https://www.AR15.Com/media/mediaFiles/997/ar-15-inside-209893.JPG

I have been 9mm since mid 2003, have not broken any pins here, but did cut tail off neutered M16 hammer about a year ago

View Quote View All Quotes

View All Quotes

Quote History

Quoted:

Quoted:
Look at your disconnector. You will see a spot where the tail of the hammer is hitting it. You will need to cut the tail of the hammer completely off or use a 9mm hammer.

Before -
https://www.AR15.Com/media/mediaFiles/997/ar-223-209894.JPG

After -
https://www.AR15.Com/media/mediaFiles/997/AR-15-hammer-bobbed-209892.JPG

Hitting trigger & disconnect,, suspect hammer bind is what cause hammer pin breakage for some
https://www.AR15.Com/media/mediaFiles/997/ar-15-inside-209893.JPG

I have been 9mm since mid 2003, have not broken any pins here, but did cut tail off neutered M16 hammer about a year ago

I've searched for more information on this, why isn't this method more commonly discussed? It seems like this would be the leading cause of doubles/auto, which is a common problem with 9mms. This also looks to me to be the cause of trigger slap.

I've suffered from occasional doubles with a 9mm hammer and unramped bolt. When I switched to an ALG QMS and had my bolt ramped the doubles stopped but now I see this wear on my gun too and it concerns me. Nipping part of the tail off the hammer is easy to do, but what's stopping me from doing it is that I've never heard of this being done before, and after searching I've not found it being discussed anywhere outside this thread.

So what the dilly yo? Nip or no nip?

[ARCHIVED THREAD] - My 9mm AR went full auto at the range today. Twice.

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