AR9 Conundrum

[bravofox]

I just put together an Aero glock mag AR9, FM bolt,BA barrel, ALG ACT trigger with standard springs, 8 oz extended buffer (based on blowback9's recommended system weight), ar15 spring, factory glock mags, DIY muzzle brake.  My issues are varied.  If I shoulder the rifle firmly or hold it against a wall, it short strokes and won't reset the trigger. if I shoulder it lightly, it will bump fire 3-4 rounds. if I don't shoulder it at all, it runs perfectly. can anyone point me in the right direction? Thanks 

p.s. I tried a carbine buffer in it, bad idea

Edited February 25 by bravofox
update

[mreed911]

I was going to create a similar thread but I'll jump in here - similar question and I'd love the advice of members.

 

I also built an Aero EPC 9mm Glock lower out with a carbine length tube, PSA 9mm bolt (14.18 oz) + FM 9mm buffer (5.47 oz, does not appear to be "longer") for a total weight of 19.61 oz.  Odin Works barrel.  Flat-wire carbine spring.  LaRue MBT-2S trigger.  Well lubed.  Lots of FTF.  Tried swapping out with my FM9 bolt (nearly the same weight 14.14 oz).  Same problem.  Cannot get it to feed consistently - even had one case of brass getting into the FCG.

 

Met up with a guy at the range running a similar put-together PCC with the Gentle Recoil setup: https://blowback9.wordpress.com/2023/05/28/9mm-ar-gentle-recoil-buffer-system-final/ - he had a KE lower (so no flat wire spring) and it was night and day difference in feel... and it ran without a hiccup.

 

Seems like it all has to do with weight, and I've seen similar advice to this all over the internet: Buffer system+bolt mass should always total 22oz or more.  If that's the case, why are most common off the shelf pieced-together setups coming in under 20 oz?

 

I've got the components (JRC longer tube, weights, heaview buffer) coming in to put together this system and see how it runs...  but I'd love to hear from the experts here around tuning these beasts.

Edited February 26 by mreed911

[Texaspaul]

what is your total weight of buffer and bolt?

Edited February 27 by Texaspaul

[bravofox]

22 oz

[Texaspaul]

1 hour ago, bravofox said:

22 oz

Are you using factory ammo?

[bravofox]

15 hours ago, Texaspaul said:

Are you using factory ammo?

no, 124 gr reloads

[Texaspaul]

know the power factor?

[bravofox]

1 hour ago, Texaspaul said:

know the power factor?

134 in a pistol

[mrvip27]

good place to start:

https://blowback9.wordpress.com/2021/10/22/9mm-ar-troubleshooting-guide-simple-blowback/

 

[bravofox]

On 2/29/2024 at 11:02 PM, mrvip27 said:

good place to start:

https://blowback9.wordpress.com/2021/10/22/9mm-ar-troubleshooting-guide-simple-blowback/

 

thanks for the link, I changed the trigger to a PSA EPT I had laying around with standard springs and got an 8.4 oz kak buffer that has moving weights (my original one was solid). that seems to have fixed the problem, if it doesn't I'll be back. 

[copterdrvr]

You did two of the things I would have recommended and that's changing a solid buffer out and trying a different trigger.

 

I've built 8 AR9's between myself and friends and they all run 100% and the commonality between all of them is I used Kynshot hydraulic buffers and CMC 9mm triggers.  They shoot powderpuff to hot ammo with no issues.

[bravofox]

this trigger started bump firing after about 80 rounds, do 9mm triggers have different disconnectors? if I get a light pull pcc trigger will it still bump fire? 

[Texaspaul]

9mm are harder on triggers than AR-15, a specific trigger for 9mm is recommended many to choose from!

 

Who knows on bump fire - haven't found cause.

Edited March 14 by Texaspaul

[OptimiStick]

11 hours ago, Texaspaul said:

9mm are harder on triggers than AR-15, a specific trigger for 9mm is recommended many to choose from!

 

Who knows on bump fire - haven't found cause.

How do the hiperfires hold up ? 

[Texaspaul]

I have 3 - two competitions and one PDI - 1000 rounds through one and no issues and the other is new.  PDI is Ok - mine has an issue with a spring falling out when installing, but other than that pulls well.  CMC PCC, Timney PCC and Rise 140 PCC work well several 1000 round through each and no hiccups 

[Malarky112]

On 3/13/2024 at 10:49 PM, Texaspaul said:

9mm are harder on triggers than AR-15, a specific trigger for 9mm is recommended many to choose from!

 

Who knows on bump fire - haven't found cause.

What’s the science behind this? 

[mreed911]

7 minutes ago, Malarky112 said:

What’s the science behind this? 

Heavier blowback, heavier bolt.

[mmc45414]

9 hours ago, Malarky112 said:

What’s the science behind this? 

 

9 hours ago, mreed911 said:

Heavier blowback, heavier bolt.

It has also been my understanding that the original 9mm Colt hammer was specific to 9mm, and the bolt was shaped differently where it would be cocking the trigger, and that is part of the reason for not being directly compatible with a 5.56 trigger group.  Additionally complicating matters is that some bolts have been revised to work with a 5.56 hammer.

 

I am no expert because mine is an old one (early adopter!) that was purchased as an upper and a lower that were built (from same manufacturer) so I assumed compatibility and didn't alter either, but I studied enough to make my decision not to.  Back then not many PCC triggers were available, by now I have shot it enough to be content with it the way it is.  If I start doing more Steel Challenge maybe I will consider it again someday.  

[Joe D]

You guys are really overthinking this. Just use a 2.5# CMC trigger. Mine actually come in a 2 lbs. Use a 5.56 carbine spring and a KynSHOT RB5015HD. My 9mm PCC shoots as soft as a MPX. My .45ACP PCC shoots softer.

[mreed911]

"Just"

 

LOL.

 

Now that I have the 5007, two spacer weights and an extended tube, I have the impulse I want with my 5" barrel (16" welded shroud).  Very soft impulse now.  Replacing the Aero ejector should be the last part to get it to work right - the aero one sits too low due to where the hole for the set screw is.

[bravofox]

the psa ept didn't work out. tried an ALG QMS, it worked ok. I changed it out for a regular old mil-spec and now it won't bump fire even if I try to. I might try a "PCC" trigger in the future, depending on if I like shooting a rifle at a pistol match lol. thanks for all the input guys

[mreed911]

I know they're hit and miss, but the Rise PCC trigger is working for me.

 

Also, New Armory just restocked their ejectors in case you need a spare or two.

[Darqusoull13]

As a general rule of thumb, building off a single manufacturer's operating system typically yields far better reliability and less troubleshooting in the long run. 

Example: JP Lower, Trigger, Bolt, Barrel and Buffer (SCS). Everything is made to work as a system and has been tested with the other parts. 

Swap whatever other furniture you want, but when it comes to operating components, stick to a single quality brand. 

[mreed911]

6 minutes ago, Darqusoull13 said:

Example: JP Lower, Trigger, Bolt, Barrel and Buffer (SCS). Everything is made to work as a system and has been tested with the other parts. 

 

I will say my early model JP has never had a problem.  It's just heavy.

[Darqusoull13]

4 hours ago, mreed911 said:

 

I will say my early model JP has never had a problem.  It's just heavy.

Mine have weighed between 5 lbs and 12 lbs with optics. You can build them as light or as heavy as you prefer.