M*CARBO Brotherhood

80% ar9 f.a. issue

Hey all. Having full auto issues with a 80% lower, 8.5" faxon barrel and mcarbo compensator, dedicated 9mm upper, and psa enhanced mil-spec trigger.
The buffer system: 8oz. solid buffer, carbine spring and tube.
The bolt group: 14.5oz. bolt(including removable weight).
I have tried the following: Bolt with weight, springco red buffer spring, results in sporadic f.a. 2-3 round bursts. Bolt with cheapo carbine buffer spring, same result. Bolt with weight with tubbs .308 spring, full auto. Removed bolt weight(weight and pin is 2.57oz.), same results but more recoil. I tried the enhanced trigger with both oem and jp enhanced reliability springs and no difference. The buffer tube is carbine length and the buffer is 4." long. The bolt head has 3/16" clearance between it and the bolt catch when retracted. When I first put this thing together, I tried an aao drop-in 3.5lb trigger, same problem. Sent it back and they sent me another, no dice. I don’t know what else try. Any ideas would really be appreciated…Thanks all.


disclaimer: I dont own an AR9

With the AR unloaded. Pull charging handle, pull trigger and hold, pull the charging handle again, let go of trigger, what happens?

Maybe check the bolt/firing pin? Seems like you swapped everything else. Make sure there’s a spring installed on the firing pin? Is the firing pin binding in the hole causing slam fire?

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Good description of your build. Did 3 AR40s last year, can easily duplicate what you experience.
Use a Millspec FCG w/ Full weight springs. Period. No enhanced, no drop in. Big heavy crunchy single stage.
Start with carbine spring.

With 1 round in chamber and empty mag, fire, do not release trigger.
Does bolt lock back?
Is hammer on disconnect?
Release trigger.
Does hammer stop on sear?
If either yes or no to above, still go to Sprinco Orange.
Repeat process with orange spring. If yes (disconnector, release, sear)
If yes load 3 and repeat.
If no to orange, go back to carbine spring, 2 rounds.
What is happening with these, a lot of bolt speed, solid buffer so no anti bounce anti dampening weights, lots of banging and slamming, and they will either
1.)Blow right by the disconnector and sear, (too much bolt speed and bounce) or,
2.) Exactly the opposite, they will recoil just enough to eject and strip the next round, but not back enough to catch the disconnect (not enough bolt speed). This is exaggerated by nice FCG’s. Also bolt speed can move so fast the mass of the firing pin will set off the next round.
Sorry about the long post, spent quite a bit of time on this.

You are trying to reach a happy space between too much and not enough bolt speed and avoid overly sensitive primers. I like Wolf in blowbacks with big floating pins.
The red spring and the tubbs don’t seem to have the right profile for that long of a buffer, they don’t compress enough.
Let me know, results.

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Edit, need to drop bolt to do disconnector/sear test. Mag out.

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Thanks a lot for the replies. I forgot to mention that faxon bolt is the second one I have tried.(yes, I have spent much more on this than originally intended). The firing pin thing is one of the first things I checked on both bolts and they checked out ok. I have and sks so I’m familiar with that. There were no pin marks on a second round primer. I will try the other stuff you guys mentioned in the morning. It’s 3:00am now. Again, thanks much for the help.

I’m assuming you aren’t using an assembled trigger group. If that’s the case let me tell you your going to have a better time of it with an assembled trigger group like this one here.

Thanks. That
may be something to look at down the road a piece.

@ENTroPicGirl I think in his original post he describes both drop in and enhanced Milspec. I tried a couple different drop ins, including one from Stern designed exclusively for blowbacks, still went full fun on occasion. It’s all about finding that sweet spot in the recoil balance and having a beefy disconnector and trigger spring

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I’m fixing lunch and then gonna go out back and see what I can see. Kind
of windy today.

I’ve built a number of lowers out of everything including a ream of office paper and about a gallon of epoxy. How ever I’ve never done one in 9mm

I’m curious if there’s a chance that there an alignment issue. With 9 mm being diminutive and power everything has to be closer to spec with littleroom for error. Might be time to apply some Prussian blue to parts and see how they are interacting with one another. Try to establish a reckoning point.

The difference is a non locking bolt and no delay whatsoever while in a rotating bolt the bullet travels down the barrel prior to passing the gas port, pressures drop, and then the chamber unlocks, so the bolt speed in a non-delayed blowback, unless controlled by a lot of weight, is crazy fast and starts way too soon, slamming into the back of the buffer extension tube and violently slamming closed, with enough force to do stuff like this.

Stripped the case, stuffed the bullet into the chamber anyway, and fired. Bullet had plenty of velocity to hit target. My chin was a little bloodied from shrapnel, shoot southpaw.

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So all I can think is to 1 lengthen the gas tube to use a smaller diameter of gas tube and or reducer three-way adjustable gas control something with more than three settings you’re gonna need something at least five or six settings lastly when all else fails going with a piston actuated bolt carrier group but even then you’re still gonna have timing issues.

I remember having issues with a DPMS 9” AR I built that had the 4 selector switch. Finding the right length of gas tube and the right diameter was proving to be an issue so I sprung for one that was just a little to short but to offset that I inserted a tube that fit inside the other crimped it softly in place it was long enough that it couldn’t go through the pigtail gas tube. From that point I had could then make finite adjustments to the gas block

No gas tube on an AR9.
It is a blow back action like in a .22 rifle.
Tuning the action is done by bolt weights, buffer weight and buffer springs.

I believe, correct me if I am wrong @GinNC No gas tube. No port. Imagine a piston recoil system with the gas port in the chamber and the spent brass as the piston, now you are getting the idea…Pistol Caliber AR’s with a gas system seem to be like unicorns, until you get to 10mm

And think how big the BCG is and how much spring (and how little powder/ bullet) is in a typical .22 semi auto…

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Exactly, just like a Sub2K, heavy bolt to control cycle speed.
Most of the felt recoil is generated by the bolt movement.

I guess I
forgot to mention that this build is a so-called “pistol” and is
limited on trigger to brace butt length. Under 13.5” inches I think.

Sorry, I
just re-read your email. I don’t have a gas tube. It is a 9mm

Difference being S2K’s and other blowback carbines and pistols were designed by engineers and others that know quite a bit about the subject, and manufacturers that had a QAQC protocol in place before releasing to the general public (With some exceptions of course), where as we come along, say “Wouldn’t a pistol caliber blowback AR be cool” and start ordering stuff from Joe Bob…


We have had quite a bit of discussion on this on other forums, not sure that is correct. Truth is I don’t think anyone knows…

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