Occasional / Unwanted 2/3/4 round burst with AR 9 build

[LSWC]

Hey guys - Fairly new build at this end and I'm seeing some random burst fire.
I noticed it within the first few magazines I put through the rifle but wanted to get in the 500 round range to see if it would work itself out.

I've been using 15 round Glock mags and I've hit burst fire conditions with all mags.

Upper & lower are Joe Bob's Spartan 9
BCM PNT Trigger
FM Products BCG
FM Products 9mm Receiver Assmebly

Has anyone else run into a similar situation? Too light of a trigger spring possibly?
Any thoughts are appreciated.

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[Medic!]

Has anyone else run into a similar situation? Too light of a trigger spring possibly?
Any thoughts are appreciated.

Yes people in prison for building what amounts to a machine gun. Then talking about it on a public forum.

 

[LSWC]

Yes people in prison for building what amounts to a machine gun. Then talking about it on a public forum.

Sure - If folks without proper licensing modify guns beyond semi-auto to burst or full auto, I can see trouble down their path.

This is a parts build with an unmodified trigger or lower that's acting up. Would like to troubleshoot the issue.
I have a friend that is licensed and does have full-auto hardware so no need to dive into that end of the pool for me.

 

[Diamondback]

Check disconnector engagement and spring?

 

[Medic!]

Hope you get it fixed.

Good luck.

 

[Diamondback]

@CountryGent recommend deletion of thread and OP should take it to a pro gunsmith.

 

[Velzey]

Only a couple things it can be. Check your sear engagement and also check your disconnector.

 

[LSWC]

Only a couple things it can be. Check your sear engagement and also check your disconnector.

I'll dive in and see if I can see anything - Trigger locks up tight every time I engage it though.
I wonder if this could have been over polished during production?
I have an email off to BCM explaining the condition - curious if they'll share any info or suggest an armorer or replacement...

 

[notazombie]

You sure it has the right buffer?

 

[LSWC]

You sure it has the right buffer?

Yes - Buffer tube, buffer & spring were sold as a kit for 9mm AR - Bolt is also made by the same company.

 

[notazombie]

Yeah, I saw your link. That's why I asked if you were sure. Wouldn't be the first time a vendor accidentally shipped the wrong part.

The reason I ask is that I know a guy who once mixed a rifle lower (with rifle buffer) with a carbine upper. It caused frequent double-taps. Swapped in the correct (lighter) carbine buffer and it's been reliable ever since.

 

[LSWC]

Just pulled the trigger for inspection - dusty not dirty and all easily wiped clean with no solvent.
Trigger & hammer springs were installed correctly prior to removal from the lower.
Disconnector spring was correctly in place & movement was good (no obstructions or binding). Sear engagement on trigger edge was shiny & polished where contact has been taking place.

I think I'll pull the LaRue MBT out of another AR and test it with this rifle.
Maybe results from this test will be in sync with direction from BCM / Brownells.

 

[No_Regerts]

I received an Olympic Arms service match rifle from an estate. I took it to the range at Ft. Lewis and squeezed the trigger only to have three rounds go down range. Everyone turns to look at me. Next squeeze sends one. Next squeeze sends one. Next squeeze sends five. Now everyone is staring at me. Packed my stuff up and left. Took it apart and it looked like maybe someone had done some polishing, probably to help out the horrible trigger, and just went a little too far.

Like @Velzey said, check the disconnect and sear engagement.

 

[Member 34319]

9mm ARs have a lot of mass moving back and forth. This is the reason some manufacturers do not recommend an aftermarket trigger. Some even make a specific trigger to be used in 9mm AR builds. Usually a solid run of the mil gritty heavy trigger is fine. That doesn't mean always. Essentially the buffer and bolt going home into battery can disengage the disconnect and cause the gun to run away.

In other words the mass is bumping the disconnect causing a run away.

It's a known issue with 9mm AR15 builds.

 

[LSWC]

9mm ARs have a lot of mass moving back and forth. This is the reason some manufacturers do not recommend an aftermarket trigger. Some even make a specific trigger to be used in 9mm AR builds. Usually a solid run of the mil gritty heavy trigger is fine. That doesn't mean always. Essentially the buffer and bolt going home into battery can disengage the disconnect and cause the gun to run away.

In other words the mass is bumping the disconnect causing a run away.

It's a known issue with 9mm AR15 builds.

Thanks for this info - it makes a lot of sense.
The buffer is heavier than normal for sure so I can see inertia of the buffer returning to rest slamming the lower and affecting trigger engagement.

I'll get a stock mil-spec trigger and drop that in, maybe a heavier spring thrown in just in case it's needed.

 

[Member 34319]

Thanks for this info - it makes a lot of sense.
The buffer is heavier than normal for sure so I can see inertia of the buffer returning to rest slamming the lower and affecting trigger engagement.

I'll get a stock mil-spec trigger and drop that in, maybe a heavier spring thrown in just in case it's needed.

Start with the regular trigger.

The heavy spring will only amplify the effect, or cause short stroking issues.

 

[Kruel J]

@CountryGent recommend deletion of thread and OP should take it to a pro gunsmith.

Why? The OP isn't suggesting he tried to build a non-NFA MG. He's having a malfunction of the rifle. And he's not the first person to have a build act up with more than one round being sent with a single trigger press. There's nothing illegal here. Just a malfunction that the OP is looking to fix.

Lighten up guys.

And goes without saying any suggestions other than getting this firearm to properly function in SA will result in this thread being locked up tighter than a bull rider's azz at go time.

 

[11Charlie]

And goes without saying any suggestions other than getting this firearm to properly function in SA will result in this thread being locked up tighter than a bull rider's azz at go time.

You have such a way with words!

 

[Member 34319]

Why? The OP isn't suggesting he tried to build a non-NFA MG. He's having a malfunction of the rifle. And he's not the first person to have a build act up with more than one round being sent with a single trigger press. There's nothing illegal here. Just a malfunction that the OP is looking to fix.

Lighten up guys.

And goes without saying any suggestions other than getting this firearm to properly function in SA will result in this thread being locked up tighter than a bull rider's azz at go time.

Ya but, his evil black gun was going fully semi auto!

 

[solv3nt]

Black rifles matter!