AR 9 double fire

[metalcruncher]

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[r1ghtw1ng]

Move the selector switch from 3rd to 2nd position.

[reverser]

First of all, did you properly function check your fire control group? Also, is this the brownell’s 9mm bcg with removable weight? What buffer and spring are you running? Did you leave the bcg weight in or remove it?

[alpha_romeo_XV]

My AR9 runs on a standard carbine buffer spring with an dedicated 8oz buffer.

[Red9]

Might be the fire control group? I had 3 psa sets that did the same thing. Double shots on the reset. Swapped them and it worked for me. Maybe try a different hammer/shear?
Plus one on buffer weight. Used one from Joe bob that worked great. Its a balance act. Getting weight and spring just right. Stock buffer will hammer the buffer retention pin. Too much weight and the ar9 will run sluggish.


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[morrcarr67]

I just put my AR15-9mm together but I haven't shot it yet. I'm hoping to get out next weekend.

I bought an 8oz buffer because that seemed to be the consensus on weight. Though I have heard that it can be hit or miss. I wish I had found this sooner as it seems to cover all the bases.

https://www.jsesurplus.com/kakar-15configurablebufferkit.aspx

[metalcruncher]

[Red9]

You have a bad hammer/shear group. Call who you got it from they should send replacement parts. At least psa sent me 3 sets.

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[reverser]

When you release the trigger, the sear engages and holds the hammer. If this isn’t happening, one of those two surfaces is out of whack.

Thanks for all the responses. I’m back home. The Brownells 9mm bcg does not have a removable weight. The buffer is a Midway purchase...AR Stoner 9mm buffet tube assy with a 5.4 oz weight.

The selector has two positions only, fire and safe.

But, during the function test of the fcg, when charging the AR9 with the trigger depressed, upon release of the trigger the hammer falls.

[caliguy93]

Dude is that an sBR?

What is your barrel length?


There is only two possible ways a double fire can happen. The hammer is following the bolt, or the firing pin is stuck in the forward position. Either one can happen intermittently.

First things first.. double and triple check the disconnector spring is in the right way. It’s the green spring under neath the disconnector and inside the trigger. One side of that spring has a bigger loop than the other side. The bigger side goes inside the trigger.

If that’s fine then check the bcg and the firing pins movement.. it should move freely.. out lots of pressure so the firing pin goes forward and see if it just falls back by turning it vertically.

You can also manually check the fire control group.. remove the upper, press and hold the trigger down. While holding the trigger, reset the hammer and see if it holds back into place. Do this many many times..

You should have a pretty good idea what’s going on by now.

The only other thing to check is that your bcg is pressing the hammer down far enough to actually reset.. you can check that by following the same fire control group test prop Ediet above but instead of pressing the hammer back down, U manually cycle the bolt.

[G-forceJunkie]

Its not going to fire if the hammer follows. What it sounds like is happening is the gunfires on the trigger pull, and fires again upon release, then the hammer follows the bolt down. The disconnector is catching the hammer after the first shot when the trigger is to the rear, but as the trigger goes forward, the disconnector releases before the sear on the hammer catches the nose of the trigger. I'm guessing the disconnector or the disconector hook on the back of the hammer is out of spec allowing the disconnector to release too soon. Or the holes for the hammer and trigger are not in the right place ( OP said its an 80 % lower). Try swapping out the hammer and trigger.

There is only two possible ways a double fire can happen. The hammer is following the bolt, or the firing pin is stuck in the forward position. Either one can happen intermittently.

[SkyHawk]

^^^ yep, holes could be out of spec location - first thing I thought when he said 80%

If you have or can borrow a drop in trigger, see if it lines up with the holes in your receiver and you can get the pins in with no drama. If not, you’re boned.

[Sputnik]

I had a similar issue with an AR22. I started getting doubles and it turmed out to be the fcg. Geissele SSA failed the function check. So did the Larue MBT2S. The hammer wouldn't stay back when the bolt was released with the trigger held down. A stock geometry trigger was the only cure I could find. I don't relish the thought of going fully automatic when I only requested fully semi-automatic.

[sigstroker]

Maybe you just got a binary trigger for an undisclosed bargain.

[ar15barrels]

But, during the function test of the fcg, when charging the AR9 with the trigger depressed, upon release of the trigger the hammer falls.

That's a disconnector problem.
Read the directions for the JP gunsmith fit trigger on how to adjust disconnector timing.
Lots of other factors effect disconnector timing including lower reciever dimensions, trigger travel stops etc...
It's not uncommon to have to correct disconnector timing when you have out-of-spec or nonstandard parts effecting the function of the disconnector.