9mm AR Pistol Issue (runway on trigger pull)

[Eric0321]

I'm building a 9mm AR Pistol and took it to the range for a test run today. It's firing double shots and a few triple shots with each trigger pull. I have a the Kaw Valley Precision BCG, 7.5oz (heavier) buffer plug and stronger pistol buffer spring installed. Upper and Lower is New Frontier Armory, LPK is BCM.

Any ideas on how to fix it?

View attachment 62055

 

[Mgderf]

Disconnector spring is in upsidedown.

 

[Eric0321]

Disconnector spring is in upsidedown.

Fat end on the bottom away from disconnector right? I'll check it. If it's wrong, BCM packaged it wrong as they pre-assemble the trigger springs and hammer spring... Like in this image of the packaging.

 

[Mgderf]

Yes, fat end down.
That was just a guess.
I mistakenly got one in wrong side up on one of my first builds, and it caused a like issue.

You might check to see if there are any burrs on anything regarding the mating surfaces between the trigger and disconnector.
For whatever reason, the disconnector is not doing it's intended job.

Do you know how to perform a disconnector function check?
Cock the hammer. Pull the trigger to release the hammer, but do NOT release the trigger.
With the trigger still pulled to the rear, cock the hammer again.

NOW release the trigger.
The hammer should catch on the disconnector.
My guess is that it is not, but I can't tell why without looking at it.

 

[Eric0321]

It passed functions check. Hmmm...

 

[Eric0321]

Forgot to mention. It isn't actuating the LRBHO when empty either. If I manually charge the BCG to the rear the LRBHO functions properly. I'm thinking that maybe the BCG isn't going back far enough to lock the hammer onto the disconnector and get caught by the bold catch but far enough to feed another round.

I could try the lighter buffer plug next outing. Or a weaker spring.

 

[Mgderf]

As a general rule, if the bolt carrier group travels far enough to feed subsequent rounds, then the disconnector should have already been actuated.
I'm guessing there's a mismatch somewhere between the hammer and the disconnector.
Sounds like you may need to swap out the trigger pack.
Don't suppose you have a spare laying around?

 

[Eric0321]

I don't I've read a lot of other forums where people say that a heavier buffer plug and spring fix it. But I'm already using heavy of both... I may have to take it to someone who knows more than me and pay to have it tuned.

 

[DoggyDaddy]

My first question when I saw this thread was going to be to ask if you were using a drop-in trigger. This same thing happened to me with one of those. I just hadn't tightened the set screws for the drop-in trigger enough, so it was essentially functioning like a binary trigger, firing on the pull, then again on the release. I see that's not the case with yours though.

 

[Thegeek]

What does the brass look like? First round have nice clean pin marks and numbers 2+3 look different? Are you sure it's firing by the hammer and not something else?

 

[Eric0321]

I didn't check the brass. I didn't put many rounds through it considering what it was doing. I'll have to take it back out and see what the brass looks like; which probably wont be until next weekend at the earliest. I'm not sure about the hammer. The firing pin has a decent spring on it inside the BCG, but it could be going forward with enough force to bounce it into striking maybe.

 

[Mgderf]

I didn't check the brass. I didn't put many rounds through it considering what it was doing. I'll have to take it back out and see what the brass looks like; which probably wont be until next weekend at the earliest. I'm not sure about the hammer. The firing pin has a decent spring on it inside the BCG, but it could be going forward with enough force to bounce it into striking maybe.

What?
The firing pin is free-floating in the AR platform.
Should not have ANY spring in the bolt carrier group.

 

[Eric0321]

What?
The firing pin is free-floating in the AR platform.
Should not have ANY spring in the bolt carrier group.

The Kaw Valley Precision 9mm BCG does, along with many other 9mm BCGs
https://www.kawvalleyprecision.com/product-p/kvp-9mm-bcg.htm

"We've also added a beefier firing pin retention spring to help lower the likely hood of firing pin breakage, that is common in other 9mm BCG's."

 

[Mgderf]

The Kaw Valley Precision 9mm BCG does, along with many other 9mm BCGs
https://www.kawvalleyprecision.com/product-p/kvp-9mm-bcg.htm

"We've also added a beefier firing pin retention spring to help lower the likely hood of firing pin breakage, that is common in other 9mm BCG's."

I stand corrected!
That's a nice looking bcg.

 

[Eric0321]

I didn't know that was a thing either until I started this 9mm build.

 

[jason867]

What?
The firing pin is free-floating in the AR platform.
Should not have ANY spring in the bolt carrier group.

Pistol primers are softer than rifle & military primers, so a free floating firing pin in a pistol caliber AR has a higher risk of slam firing, thus a spring to help prevent that.

Also, there is no rotating bolt to slow down the movement at the last second, or to hide the firing pin behind the breach face.