9mm AR not running FA

[Mustafa]

I've got a 9mm upper and magazine block that I'm running right now but it is not cycling properly on FA when suppressed. It shoots FA fine without the suppressor, and semi auto fine WITH the suppressor. On FA it fires the first round, and chambers the next but won't fire it. It feels like the hammer has already dropped on the bolt, but did not fire the round. Any ideas what is causing this and how I can fix it?
Thanks all!

 

[LRRPF52]

Bolt bounce. You need a different buffer. What buffer are you currently using?

On SA, you can't pull the trigger fast enough to induce the malf, but on FA, the bolt bounces off the breach just as the hammer impacts the firing pin, without contact to the primer.

Bolt bounce is addressed in the FA AR15 family with dead blow effect from heavier buffers that have a series of reciprocating masses to impact forward within the buffer to keep the carrier (in this case bolt) forward.

The Colt 9mm SMG buffer has a very heavy body, with minimal internal weight, whereas the carbine buffers for 5.56 carbines have lightweight aluminum bodies, with series of steel and tungsten internal weights.

 

[ZombieMonkey]

As mentioned, get a different buffer. Either something filled like a spikes ST-2 or other similarly weighted buffer. LRRPF52 is spot on in his diagnosis unless you've got something else crazy going on (highly doubt it).

 

[Mustafa]

Thanks! Great info! I've got an Eindyne hydraulic buffer in it now (slower RoF for 5.56). I'll try a few different buffers to see if I can get the problem to go away.

One question though, why does it work FA without the suppressor? Is the BCG traveling that much slower without the back pressure?

 

[ZombieMonkey]

One question though, why does it work FA without the suppressor? Is the BCG traveling that much slower without the back pressure?

Yeah, it really can be enough to get it to not cycle. I deal with the opposite issue with my SPR where suppressed it cycles flawlessly, but unsuppressed it is single shot. The differences with the can on vs off can be enough to make a problem pop up when you're right there on that edge.

 

[biggin69]

My FA 9mm Colt SMG runs great suppressed using a specific 9mm buffer. It is heavier than an AR15 H3 buffer. Look for a buffer specific for 9mm.

 

[ORD]

With your setup, OP, you are running into a couple of different, but closely related problems...all stemming from your buffer.

First, a 5.56/.223 buffer is TOO SHORT to run in a 9mm AR. By running such a short buffer, you are allowing the bolt entirely too much rearward travel which is a recipe for malfunctions, not to mention broken parts in your lower (especially bolt catches).

Second, as previously noted, your average 5.56/.223 buffer is WAY TOO LIGHT for use in a blowback 9mm AR. Your basic hydraulic carbine buffer weighs in somewhere around 4oz (give or take a little). A 9mm AR buffer (the standard Colt buffer for example) weighs in at 5.5oz (with the "A" and "B" model Colt buffers weighing 6.3oz and 7.1oz respectively).

For use on a FA AR-15 9mm, I would recommend at a minimum running the dedicated factory 9mm buffer, and I would personally, having run the @#$% out of one over the years now, would move on up to one of Clint Butler's (a.k.a. - Slash) 9mm "9Q-T" buffers which were designed and built from the ground up for use on suppressed 9mm ARs. See here for more info: Heavy Buffers | Products

 

[wolfgarrudel]

The Enidine hydraulic buffer will not work in my FA M-16. I put an H buffer in it from Bravo company and my 5.56 upper and 9mm Olympic 9mm work flawlessly.

 

[Mustafa]

Well the Enidine buffer works great FA with 5.56, it just doesn't work with the 9mm FA. But I'll try another buffer that I have and if that still doesn't work. I'll buy a specific 9mm buffer.
Thanks for all the help guys!
I"ll let you know what the end result is!

 

[Mustafa]

Well, the other buffer I had laying around didn't help. Got an order in for 9QT and Wolf spring. We'll see if that does it!

 

[tipper]

FA=no can

 

[ORD]

Well, the other buffer I had laying around didn't help. Got an order in for 9QT and Wolf spring. We'll see if that does it!

I think you'll be pleased with the 9Q-T from Clint. Its an excellent product, especially when run with the stiffer/heavier Wolff XP spring.

Let us know how it goes, Mustafa!

 

[eternal24k]

An alternative is also the vltor a5, just an FYI.
I went from extended SS buffer to using a spacer to an A5



Sent from my HTC One using Tapatalk

 

[Mustafa]

Well, I just got my first chance to try out the new buffer and spring. Maybe a little better, but still not what I want. Now I get a TON of FtE where the empty brass comes out of the chamber, but doesn't clear the gun so it "stove pipes" essentially. Strangely, I got two nice long strings of FA when I got pissed and hip fired if in frustration!
Any ideas for next steps?
Other things to try?

 

[ORD]

Well, I just got my first chance to try out the new buffer and spring. Maybe a little better, but still not what I want. Now I get a TON of FtE where the empty brass comes out of the chamber, but doesn't clear the gun so it "stove pipes" essentially. Strangely, I got two nice long strings of FA when I got pissed and hip fired if in frustration!
Any ideas for next steps?
Other things to try?

Weird! I thought that would have fixed the problem in its entirety.

Did you by chance try running it with the non-XP (Wolff), standard AR-15 carbine spring you were running with your Endine hydraulic unit?

At this point, you might try giving Clint a call and having a chat with him about your continued failures with your 9mm in FA with his 9Q-T buffer and see if he can offer any insight. He's forgotten more about ARs than many will ever know!

 

[Mustafa]

Probably a good idea. I'll try hitting him up. Thanks!