9mm AR-15 problem.


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Carbon_15
March 10, 2013, 01:12 PM
I recently bought a RRA 9mm upper for my AR. The rifle has functioned normally through sever hundred round before the upper swap. In 9mm configuration I basically have a single shot. The rifle will fire, ejects, and loads a fresh round, but the hammer follows the bolt forward.
I have the recommended DPMS rounded hammer, everything else in the lower is standard 223 gear.
Any ideas?

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Mamertine
March 10, 2013, 02:00 PM
I have a 9mm AR. When I was shopping for one I considered building one and did some research. They need a different hammer and I think springs in the trigger group also.

Welding Rod
March 10, 2013, 05:15 PM
Maybe short stroking and not bringing the hammer far enough back to catch?

Shooting full power loads?

I don't know if the buffer and spring is supposed to be the same as the ones used for 556, but I would check into that.

bikemutt
March 10, 2013, 06:34 PM
Did the upper come with a bolt?

Carbon_15
March 10, 2013, 09:05 PM
I was shooting + p loads. Tried some more today...same issue. Switched to standard velocity loads and the problem went away...odd.
Yes...it's a RocknRiver upper and it came with a bolt.
It's defiantly not short stroking

LJH
March 10, 2013, 09:26 PM
Are you running the 9mm buffer?
If not that would cause the problems you have mentioned.

chad1043
April 6, 2013, 01:01 PM
I would also say the Buffer might be the problem. What weight buffer are you running?

Walkalong
April 6, 2013, 01:05 PM
They need a different hammer and I think springs in the trigger group also.They need a hammer with the rounded configuration vs the notch. Springs are the same.

http://www.thehighroad.org/showpost.php?p=5378346&postcount=5

I have the recommended DPMS rounded hammer, everything else in the lower is standard 223 gear.
Any ideas?So your good to go. Sounds like underpowered rounds.

jerkface11
April 6, 2013, 01:09 PM
They make a 9 mm weight buffer.

HJ857
April 6, 2013, 01:53 PM
I had the exact same problem with a CMMG upper. However I had a RRA NM trigger in it and it was simply not tall enough for the bolt to re-set it.

I swapped out NM for an ALG Defense ACT trigger and all the problems went away. Don't discount the trigger as your problem yet. See if you can find comparative measurements for your trigger vs. full size mil-spec triggers. The 9mm runs a very close tolerance and that's definitely the first place to look.

EDIT: sorry, I said trigger, but what I'm talking about is actually is the height of the hammer itself. I replaced the entire RRA trigger/hammer assembly with an ALG assembly.

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