AR9mm troubleshooting advice sought

Scsmith42

Scsmith42

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A few years back I bought some components from a CFF member for an AR9 PCC build. I recently completed it, using a Maxim Defense CQB Brace. It's a fairly short barrel - less than 10". The brace came with a JP Silent Capture Spring system, which was for a .223. When I first shot the assembled gun, I experienced something that I've never been through before, and that is that when shooting, the trigger is recoiling so hard against my finger that it actually stings. This happens with a variety of ammo weights - 115, 124 and 147 grain practice type ammo.

Now, I've shot a tremendous variety of firearms - pistols, full auto subguns, belt feds, and assault rifles, and I have never experienced such a sharp recoil against my trigger finger. It's so bad that I don't enjoy shooting the gun, and that's a first for me.

Upon the advice of Maxim Defense, I purchased their pistol caliber buffer and spring to replace the JP one. What I bought is the heaviest one that they make. Unfortunately it did not solve the problem (but it may be slightly better)

The gun is feeding ammo properly, which to me indicates that the buffer and spring are not too light. But I'm at a loss regarding troubleshooting what is causing the sting against my finger. I would think that two weak of a buffer would have the opposite effect; ie the gun would slam against my shoulder - not moving forward and slamming the trigger so sharply against my finger.

Has anybody else run into a similar situation and been able to isolate the cause?

Thx.

Scott
 
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A few ideas
1. Do you have a mil-spec trigger to replace the one on the firearm? Just to see if its the trigger pad?
2. The recoil felt on the trigger would give you some room for reset, I ASSUME the reset of the hammer MIGHT be causing the discomfort.

John
 
@Scsmith42 The silent capture system for an AR15 is 5.3oz. The silent capture system for a 9mm AR is 11.2oz. I would say its the silent capture weight as the 9mm is almost double

Nevermind - I see it happens with the Silent Capture removed
 
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@Scsmith42 I have a Foxtrot Mike Colt pattern bolt you can have for shipping cost or get some other way, won't swear it'll fix the problem, but a cheap test piece if you want it.
It didn't agree with my "mil-spec" upper and charging handle. Ran fine, just tended to wedge tight when charging. Side rant FM's "great customer service" told me their rejection rate was less than 0.0x% so it wasn't an issue with their bolt, no problem, other brand works, will never buy anything from them again.
 
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Beef15 said:
Colt pattern? Wonder if you got an old unramped bolt?
Some reading https://blowback9.wordpress.com/202...need-for-my-9mm-ar-hammer-bolt-compatibility/
Click to expand...
That is some good insight! Yes, I'm using Colt mags. Years ago I had the opportunity to shoot and original DEA9mm and really liked it. So when I was thinking about building a 9mm AR platform I opted for Colt instead of Grock.

I'll get the gun out later and look at the bolt, but this indeed could be the problem.

Many thanks to all who shared advice thus far, and for your kind offer to ship me a bolt.

I'll report back after I retrieve the gun from the safe.
 
What trigger? Is your disconnector spring big end down? Built one years ago and had the same issue, played around with disconnector hook length and found it solved the problem. Mine was a full blowback gun not a gas or hybrid system. Also with blowback VS gas your not losing any gas pressure via impingement so possibly made need a heavier buffer, I ran a 8oz if memory serves me correctly.
 
Every colt 9 smg including mine trashed trigger pins Colt at one time moved to a harder nickel? Pin it didn’t help. 1-2000 rds bet on it- however as others said ramping the bolt helped. As a side note we had colt 9mms post samples come in with different hammers sometimes but never a ramped bolt .
 
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Thanks all for the feedback. I finally got some shop time today and took some pix of the bolt. To answer some of the questions, the gun has a standard millspec fire control group and it is a direct blowback. It has a 7.25 oz buffer, which is the heaviest one that Maxim Defense offers and the one that they recommend for a 9mm pistol.

Here are some pix of the bolt. It's a New Frontier Armory AR-9 Standard 9mm BCG. I haven't been able to come up with any close up pix comparing ramped versus non-ramped 9mm bolts to know what to look for. I've reached out to NFA to see if they have any suggestions regarding the cause of the problem, but have not yet heard back.


Bolt 1 C.jpg

Bolt 2 C.jpg
 
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It sounds like bolt and or buffer problems
I run dedicated 9mm lowers (colt mags), Toolcraft BCG v2, Macon Armory 9mm Deadblow Buffer,
and their 9mm Guaranteed to Feed barrels no issues. "Rudy" does great work
Macon Armory: https://maconarmory.com/collections/all
then for some of the uppers are SMF Tactical 9mm, their 9mm LPK or just a decent LPK
Here's Some reading material from Droppoint at Arfcom:

Lots of 9mm AR information - AR15.COM

Firearm Discussion and Resources from AR-15, AK-47, Handguns and more! Buy, Sell, and Trade your Firearms and Gear.
www.ar15.com www.ar15.com
and as posted above in an ealrier post:
blowback9.wordpress.com

Blowback9.com

No-BS information for the 9mm AR.
blowback9.wordpress.com blowback9.wordpress.com

-Snoopz

toolcraft 9mm BCG.jpg
 
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I had bad trigger slap on a few of mine. Most due to the trigger group.
Reolaced the trigger to a different type or style and it was corrected.
Happened with cheap triggers and a few name brand.

If that is what you are getting try a different trigger group.
 
The JP system requires that the center of the bcg be hollow (that rod has to go somewhere in recoil). Did you put the weight back into the bcg when you swapped to a heavy buffer and spring?

No idea if this would cause the problem.

BTW, do like the JP system.
 
I'm thinking it's related to the trigger assembly. I'm running the POF in two of these, and we just happen to have someone in the BST selling one. https://carolinafirearmsforum.com/i...-pof-drop-in-trigger-4-5lb-curved-rdu.115036/


Also, there's this photo I have from Foxtrot Mike that explains the difference between their Glock and hybrid Colt bolts. My understanding was the below photo only spoke to the feeding issues that exist between different generations of bolts and magazine types. I have run one of each in identical guns and the one with the Glock only bolt needed a little dremel work to even close properly. I'm using a mag well adapter that takes Beretta 92 mags and is intended to be used with a Colt bolt.

1653908407405.png

I have a few of these around the house, and I'll dig for them if you need pictures, but I'm lazy enough that I'm going to ask the OP to ask before I get off my butt.
To me it looks like the bolt the OP has is a Glock bolt. The shipping cost offer to get a Colt pattern one from @Beef15 is one I would take him up on.
 
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JimB said:
The JP system requires that the center of the bcg be hollow (that rod has to go somewhere in recoil). Did you put the weight back into the bcg when you swapped to a heavy buffer and spring?

No idea if this would cause the problem.

BTW, do like the JP system.
Click to expand...
I have not put the weight back in. That’s my next step.
 
Timfoilhat said:
I'm thinking it's related to the trigger assembly. I'm running the POF in two of these, and we just happen to have someone in the BST selling one. https://carolinafirearmsforum.com/i...-pof-drop-in-trigger-4-5lb-curved-rdu.115036/


Also, there's this photo I have from Foxtrot Mike that explains the difference between their Glock and hybrid Colt bolts. My understanding was the below photo only spoke to the feeding issues that exist between different generations of bolts and magazine types. I have run one of each in identical guns and the one with the Glock only bolt needed a little dremel work to even close properly. I'm using a mag well adapter that takes Beretta 92 mags and is intended to be used with a Colt bolt.

View attachment 479660

I have a few of these around the house, and I'll dig for them if you need pictures, but I'm lazy enough that I'm going to ask the OP to ask before I get off my butt.
To me it looks like the bolt the OP has is a Glock bolt. The shipping cost offer to get a Colt pattern one from @Beef15 is one I would take him up on.
Click to expand...
Good info, thx. I’ll check that out.

Many thanks to all for the feedback.
 
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I had the same trigger pain in my finger from an out of spec oak armory reworked RRA two stage trigger. It was bad enough if I held the trigger back it would stop the bolt from cycling correctly.
 
JimB said:
The JP system requires that the center of the bcg be hollow (that rod has to go somewhere in recoil). Did you put the weight back into the bcg when you swapped to a heavy buffer and spring?

No idea if this would cause the problem.

BTW, do like the JP system.
Click to expand...

I finally got some time today to work on the PCC. I sourced another BCG from New Frontier Armory (NFA), and it was slightly different than the original NFA one that was installed in the gun. The new BCG-9 included the weight in the center of the BCG, which was lacking from the original BCG. I'm thinking that one of the BCG's is for a Grock lower and the other for a Colt; hence the slight difference.

BTW, the upper is an SLR-B9 from SLR Rifle Works.

The new NFA BCG-9 was a very tight fit in the receiver, to the point where I could not cycle the bolt sometimes (it was stuck in place). After thoroughly cleaning the receiver and well oiling the new bolt, I figured that I'd try it to see if it would wear in. The good news was that the trigger slap was much better; the bad news was that it would frequently jam.

So, I removed the weight from the center of the NFA BCG and installed it in the old BCG, and it's now cycling corrctly. Trigger slap is significantly reduced, but still present (but not to the point where it is uncomfortable to shoot the gun).

Patrick Ward, gunsmith of New Frontier Armory and I have corresponded about the problem. He confirmed that all of their bolts are ramped, and that trigger slap is not an uncommon problem to encounter. He recommended a trigger from Hyperfire (PDI FCG) as one resolution, or that I reduce the mass and depth of the current hammer in order to reduce the inertia (but not the height).

So that's where we are at. I'll probably source a second hammer and play around with it in the milling machine to see about reducing the mass.

Many thanks to all that have shared advice.

Scott
 
This thread is a good read for anyone considering a Franken-build blowback “AR-9”. They are often problematic for a whole slew of reasons. I got out of these way back when I got tired of experiencing tolerance stacking on my own dime. OP hopefully you’ll get the carbine where you want and it stays there. BTW my experience w SLR has always been great….have you considered reaching out to them and get their input?
 
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92G said:
This thread is a good read for anyone considering a Franken-build blowback “AR-9”. They are often problematic for a whole slew of reasons. I got out of these way back when I got tired of experiencing tolerance stacking on my own dime. OP hopefully you’ll get the carbine where you want and it stays there. BTW my experience w SLR has always been great….have you considered reaching out to them and get their input?
Click to expand...
Yes, I spoke with them when I was originally seeking lower components for the gun. Great folks!
 
Update. I’ve made a couple more changes which have solved the trigger slap problem. First was the installation of a stiffer hammer spring. The original hammer spring had been bent back somewhat by the friend who assembled the trigger pack (why I don’t know). We replaced it with a stiffer, unbent spring and that helped with the slap (but did not eliminate it).

I’ve been hesitant to lighten up the trigger (concerned with potential to crack), so instead I opted to increase the amount of weight added to the center of the bolt. The original weight was mild steel and weighed 2.4 oz. It was held in by a roll pin. I opted to make a new weight from tungsten carbide. Pure tungsten would have been my first choice, but it is prohibitively expensive. Tungsten carbide weighs almost as much as pure tungsten, but is much less costly and more commonly available.

I found a 6” long, .625 diameter piece of tungsten carbide round bar for $45 on e-bay. The biggest challenge now was how to cut it, since tungsten carbide is what high end machinist tooling is made from. It is extremely hard and very brittle. I was able to shear a piece off by using an abrasive cutoff saw to start the cut, and then break the remainder off. In order to be able to grind it effectively, I heated the end cherry red with a rosebud torch and ground it successfully while hot (since the grinding wheel just skipped off the surface when it was cold).

The original roll-pin holes in the BCG were drilled out and tapped for 5/16” hex head set screws, which were ground down to fit. I ground slots into the sides of the weight in order to provide a depresion for the set screws to engage.

Here are a couple of pix. The first one is of the tungsten carbide being ground while red hot. It’s wrapped in a piece of leather to keep the vise jaws from marring it.

A9DEC93C-5432-438C-951B-585B75403786.jpeg



The second pic is of the counterweight installed in the bolt with the set screws. The new weight is 5.8 oz, so I’ve increased the weight by a factor of 2.4X. Some test firing today indicates that the trigger slap problem has been solved. The set screw was slightly loose in the pic below; after test firing everything was degreased and the set screws installed with red Loctite in a manner to clear the exterior diameter of the bolt.



E19981E7-40C1-4FC2-A486-ECEF6B36AAE3.jpeg


Next test is to try the gun out with a suppressor attached and see if it still runs reliably. I may need to decrease the amount of bolt weight somewhat, or experiment with a lighter buffer and or spring.

Many thanks to all who shared advice. The journey continues!
 
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