AR9 trigger help.

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Baron85

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Mar 18, 2012
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#1
I built an AR9 some years ago using a quarter circle 10 reciever, barrel, and jp bolt. I couldn’t get my favorite Larue MBT triggers to work so it ended up with a milspec from the spare parts bin.

I have put a few hundred rounds through it and it started to burst fire at the range so I pulled it apart and inspected the trigger but didn’t see any issues. I replaced with another milspec and same thing happened.

I see there are PCC specific triggers, is that because normal AR triggers don’t work? Is there something else I need to look into as to why it keeps burst firing?

I have built a handful of ARs without issue so I don’t believe it’s an assembly problem, but anything is possible.
 
#2
Make sure your bolts FP isn't "stuck" from gunk or a pierced primer disc.

Some triggers don't "work" with every 9MM PCC bolt... on the underside of the bolt, the ramp for the trigger uses for "cocking" can be to short of an angle ( or to long ) or the ramp isn't "tall" enough to reset triggers with a less than USGI hammer profile. ( Thats why your MBT didn't work, and it doesn't take much )

I would also suggest checking all the fire control parts... looking for any battering ... sometimes the hammer can be reset so forcefully, that it can pound on the FCG parts.

Also, what style 9MM PCC, Glock mags ? MP5 Mags ? ....Colt Pattern ? ... and caliber ?

EDIT... and what buffer weight, length ?
 
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#3
bfoosh006 said:
Make sure your bolts FP isn't "stuck" from gunk or a pierced primer disc.

Some triggers don't "work" with every 9MM PCC bolt... on the underside of the bolt, the ramp for the trigger uses for "cocking" can be to short of an angle ( or to long ) or the ramp isn't "tall" enough to reset triggers with a less than USGI hammer profile. ( Thats why your MBT didn't work, and it doesn't take much )

I would also suggest checking all the fire control parts... looking for any battering ... sometimes the hammer can be reset so forcefully, that it can pound on the FCG parts.

Also, what style 9MM PCC, Glock mags ? MP5 Mags ? ....Colt Pattern ? ... and caliber ?

EDIT... and what buffer weight, length ?
Click to expand...

I went through this when I built 3 AR9's 5-7yr ago.

Larue confirmed their triggers won't work jn AR9..


ALG ACT trigger works great...
 
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#4
I have a couple QC10 AR-9s I built, and they have specific needs. I think QC10 has on their site advice about triggers, buffers, springs etc.
Remember that these are simple blowback. If the trigger doesn't lock back it can follow the bolt down doing bursts. If it fires out of battery you may find your magazine rocketing out of your lower.
Making sure trigger locks and the bolt doesn't short stroke or bounce hard causing issues is important.

I have had no trouble with mine, but I followed parts lists known to work together.
Both have ALG ACT triggers, QC10 bolts and 5" TROS 3-Lug barrels. One has a B5 SOPMOD stock and Blitzkrieg hydraulic buffer made for AR-9 and the other has MVB ARC stock and 5.3z buffer made for 9mm.
 
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#5
It uses Glock mags, 9mm, and I believe I went with the JP silent buffer but will have to check when I get home.

It has been sitting un used for a long time and recently peaked my interest again and try and figure out the problem.
 
#6
I’ve ran thousands of rounds on my ar9 using all kinds of triggers.
-Mil spec work fine
-Tac Con 3mr in it probably 10/11 years ago ran fine
-Franklin binary ran fine


I have a Foxtrot Mike dual charging upper as well and it won’t work with a Larue MBT or any type of notched hammer. The front of the bolt on Glock bolts hangs up on the notch.

They are hard on parts cause it’s a lot of mass moving back and forth in there. I’ve burned up a disconnector spring or three, as well as the firing pin spring. Check the FP spring or make sure it isn’t busted and allowing the firing pin to slam into the next round as it goes home as well. Probably a long shot on that happening, but the world is crazy

IMG_7961.jpeg
 
#10
I have a CMC trigger in a QC10 lower that runs very well, used primarily in USPSA.
I have at least 5k rnds through it (probably closer to 10k) and never had a trigger related issue.
 
#11
OP- Help, my faux sub gun trigger is burst firing…

Everyone else- Lucky…

Oh, I have a Rise armament trigger in my 9mm AR. No issues.
 
#13
hlee said:
OP- Help, my faux sub gun trigger is burst firing…

Everyone else- Lucky…

Oh, I have a Rise armament trigger in my 9mm AR. No issues.
Click to expand...
My AR-45 used to bump fire quite well, but it would ONLY do it when I didn't want it to. In matches. Which would usually cause the RO to stop me and I'd have to explain it wasn't defective, broken or unsafe, it was just bump firing. :rolleyes:
 
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#14
hlee said:
OP- Help, my faux sub gun trigger is burst firing…

Everyone else- Lucky…

Oh, I have a Rise armament trigger in my 9mm AR. No issues.
Click to expand...
Haha yeah, it would be fun if I knew it was going to do it and could control it. Not so cool when you’re not expecting it.

Hoping to play around with it at a usps pcc match at some point but wouldn’t feel comfortable trying until I get this issue figured out.

And as far as bump firing being the cause I doubt it, I had bump fire issues on a 308 AR with timney trigger set really light. I have made sure I’m pinning the trigger back on this rifle and it’s still bursting.
 
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#15
FYI- bump fire and bursts can be very dangerous on these. Sooner or later you will get it to fire out of battery. The gasses will ruptured the case and can cause damage to the gun, shooter or those around.
This is from an Olympic Arms upper in 9mm from about 25 years ago.
17428642854442291780691209588447.jpg
.
 
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#17
hlee said:
Is that OOB detonation, or early extraction? Or maybe even a not fully supported chamber?
Click to expand...
It was OOB, needed a heavier buffer and the trigger would bounce when the buffer hit the back of the tube hard. Result was hammer following bolt and hitting firing pin when cartridge not fully in.
For those who don't know- on a regular AR with locking bolt a strike on the FP before bolt is locked will not fire a cartridge as the FP is blocked until the bolt rotates.
On these simple blowback pistol caliber guns, the bolt has no safety feature or rotating bolt so the firing pin can strike before bolt is fully closed.

I had two OOB out of thousands of rounds. Once I put in heavy buffer, it was fine
 
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#19
Old Man with Gun said:
It was OOB, needed a heavier buffer and the trigger would bounce when the buffer hit the back of the tube hard. Result was hammer following bolt and hitting firing pin when cartridge not fully in.
For those who don't know- on a regular AR with locking bolt a strike on the FP before bolt is locked will not fire a cartridge as the FP is blocked until the bolt rotates.
On these simple blowback pistol caliber guns, the bolt has no safety feature or rotating bolt so the firing pin can strike before bolt is fully closed.

I had two OOB out of thousands of rounds. Once I put in heavy buffer, it was fine
Click to expand...
whether it's on a pcc or not, your statement is true about ar design
 
#21
Slowing down the db force solves alot of issues with these systems.

Some years back I tried out a hydro buffer with a tubb 308 flat wire and never looked back. It is soft and mimics the feel of my buddies mpx, and just easier on all parts overall.
 
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#24
I went through the rifle last night.

Firing pin wasn’t stuck and spring was in good shape.

I do have the JP silent buffer and bolt.

Trigger didn’t seem to have any damage. The mating surfaces looked smooth and nothing broken.

I’m going to order a trigger and see if that resolves my issue. Either the ALG ACT or the CMC PCC.
 
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#25
These blowback PCC’s are so popular, but few people actually tune them correctly. Or they just run them the way they came out of the box, which shouldn’t be a problem. Except manufactures are often underestimating the buffer weight.

I highly recommend you increase your buffer weight and slow your cyclic rate. Even with low power factor cheap 115g 9mm ammo (worst case scenario), I have no problem with cycling and locking back on empty with a 8in barrel and using a 10oz buffer, a bolt with an internal weight and the standard Law Folder 2.2oz carrier extension.

Buffer I highly recommend is the Midway USA 9mm extended heavy buffer Midway 9mm buffer.

Website I highly recommend and videos to watch how a 9mm PCC cycles and how to better control dwell/cyclic rate. This will directly relate to solving 9mm ar platform trigger issues.
Blowback 9


In summary, I run a…
flat wire standard strength buffer spring
10.8oz buffer
2.2oz law folder adapter weight
Fail zero 9mm bolt. (Internal weight modified, and compatible with law folder)
Geissele SD3G trigger (not made for blowback, due to some guns having harsh bolt speed/cycling)
Aero EPC platform with 8in barrel.

This combo runs perfectly, always locks open on empty, smooth recoil impulse, cyclic rate is slow and reliable compared to cheap AR9’s with light weight buffers. Proof is that I haven’t broken the Geissele trigger even tho it’s a blow back operation. About 1k rounds down the tube. Since the cycling action has an appropriate amount of weight, it is only about as harsh as a regular ar cycling on the trigger.
 

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#26
Great info. I will weigh my buffer and bolt tonight. Reading on JP’s website it seems that they have changed their bolt and buffer maybe. I would describe the recoil of mine as harsh, which I was quite surprised at.

It’s a 5” with a suppressor.
 
#27
Baron85 said:
Great info. I will weigh my buffer and bolt tonight. Reading on JP’s website it seems that they have changed their bolt and buffer maybe. I would describe the recoil of mine as harsh, which I was quite surprised at.

It’s a 5” with a suppressor.
Click to expand...
I shopped around a good bit and planned on using the JP system. However then I found all the info on Blowback9. If I remember right, the JP was no where near heavy enough.
 
#28
That makes sense, because mine feels like the bolt is slamming hard into the back of the buffer tube in recoil. I just figured that’s how it was assuming that JP would be perfect using their bolt and recoil system.

I will weight it all tonight.
 
#29
Also, take caution with the frequent recommendations to run a heavier style buffer spring. While that may work, it is not optimal and is indeed still masking the signs of an undermassed recoil system. A proper recoil mass (bolt + buffer in the 22+ oz range) will run just fine on a standard spring weight, also decreasing wear associated with running these heavier springs. Again, the bolt face is just slamming into the barrel breach face everytime it cycles.
 
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#31
Baron85 said:
Do you have experience with the hydraulic buffers vs the standard buffers?
Click to expand...
I do not. However they do not particularly appeal to me. One more thing to fail and I get the feeling it’s similar to a heavier buffer spring. It’s still masking a symptom of a less than optimal setup. Maybe I’m lucky or a rare one, but I thoroughly enjoy my AR9 with 26.8oz of bolt mass and a buffer with a sliding weight. It functions very well and just feels right. So I really haven’t had to go seeking for solutions to these problems.


Another thing I would have concerns with a hydraulic buffer…

Using the buffer I listed above, my bolt only travels about 1/16in behind my bolt stop. Quite similar to the clearance observed in a standard AR15. With a blow back system, if your bolt travels too far behind a BHO, when the bolt returns it has too much force and is known to break off the bolt stops from this force.

Would a hydraulic buffer travel too far rearward during recoil? Would it then hit the bolt stop harder from the further travel? Would the release of the hydraulic action “propel” the bolt forward with increased speed?
 
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#32
MurseFlatLine said:
I do not. However they do not particularly appeal to me. One more thing to fail and I get the feeling it’s similar to a heavier buffer spring. It’s still masking a symptom of a less than optimal setup. Maybe I’m lucky or a rare one, but I thoroughly enjoy my AR9 with 26.8oz of bolt mass and a buffer with a sliding weight. It functions very well and just feels right. So I really haven’t had to go seeking for solutions to these problems.


Another thing I would have concerns with a hydraulic buffer…

Using the buffer I listed above, my bolt only travels about 1/16in behind my bolt stop. Quite similar to the clearance observed in a standard AR15. With a blow back system, if your bolt travels too far behind a BHO, when the bolt returns it has too much force and is known to break off the bolt stops from this force.

Would a hydraulic buffer travel too far rearward during recoil? Would it then hit the bolt stop harder from the further travel? Would the release of the hydraulic action “propel” the bolt forward with increased speed?
Click to expand...
There are hydraulic buffers for pcc's. Works/designed to be same compressed lengths of a pcc buffer.
 
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#34
There was a time when running a PCC was a pain in the butt. Bolts were cut for either glock mags OR colt mags, but not both. These days, most bolts will accommodate both.

The bolt profile also had an effect on trigger compatibility. What I found the safest route was to stick with milspecish trigger groups. Meaning triggers that use a full power hammer spring. I always stuck with either geissele or some form of schmid. I never had a problem with either.

As for the rest of the build, stick with standard buffer springs. The only reason to go with a heavier spring is if you need it to help with recoil recovery. The spring weight has little to do with the actual closed bolt lock time, for lack of a better term.

When it comes to buffers, avoid solid buffers. You want to use a buffer with sliding weights. This will help with bolt bounce.

A good way to setup a 9mm pcc is to use a vltor A5SH buffer in a standard carbine length buffer tube and grab yourself a few tungsten buffer weights. This will give you a lot of flexibility in tuning.

I do have friends that run hydraulic buffers and they swear that it makes the rig run more smoothly. To me it just feels like the rig is sluggish. I like my rigs to run snappy.
 
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