So I took the new AR 9MM to the range this morning for the first time. Using ASC mags, I shot the first 20 rds (in 5 round increments) with no issues. After that, every 5 rounds would include one round where the bolt would close following the previous shot, pull the trigger- nothing. Rack the charging handle, eject the unshot round, back in business. What do you guys (and gals) think? Thanks!

How about some pictures to start off with?  What ammo are you using?

Need to see primer strike on both the fired and non-fired round.  Trying to determine if the issue is light strikes but the initial though is your ammo may be the culprit.

Pics:






It sounds like the hammer issue may be the winner. Let me investigate more thoroughly. There are primer strikes on the unfired rounds.

How would I fix the hammer following problem?

I would double check that the hammer/trigger parts are installed correctly. Have read that being the cause of similar symptoms before.

Thank you Steve. Look's like it's back to the range for me.

Number your mags so you can see if one is causing issues.

MAHA

Knowing what ammunition you were running would help.  Was it commercially made by a reliable firm, like Winchester White Box bulk, or was it "something else"?  The amount of power needed to cycle the standard 9mm AR isn't huge, but if you have am extra heavy buffer and/or spring, you might just be slowing things down a little too much, especially once the gun gets dirty.  Your statement that the first 20 rounds went without problems makes me think that it got dirty enough to slow down extraction enough that the bolt didn't go quite far enough back to fully cock the hammer.

A blowback 9mm AR carbine is a filthy beast, and it gets filthier every time you fire it.  "Tuning" the system with a stiff spring, beefy buffer or other tweaks, especially without having had any real problems without these tweaks, just confuses everything.  Use a stock buffer and spring and see if you get these stoppages.

Ammo was Remington UMC. I have not done any tuning. Using a stock carbine buffer and spring I had laying around. Will number the mags.

Range report: I brought 3 more kinds of ammo- perfecta, 2 kinds of PMC. Still having issues, though they seem to be getting better. Tried both mags, one ran better than the other. I am going to order a Mec Gar mag for testing. Same issue as before, very light strikes on un-fired rounds. I'll post pics when I get home. Any thoughts?

Misc pics seen, but nothing of the lower that has any value...

Are you using an extended 9mm buffer?  I had a similar issue with a full-auto 9mm, the ramped bolt and extended buffer wouldn't allow the hammer to travel back far enough to catch the auto sear.  I had to trim about 1/16" off the back of the bumper on the end of the buffer.  If your using a standard carbine length buffer this won't apply, but your pistol may be short stroking, try a lighter buffer or reduced power spring.





I have also found that the ASC followers require a little fine tuning for best feeding.  ASC mag mod thread




 
 

If I'm understanding your symptom correctly your having issues with the hammer not being retained by the disconnector/secondary sear every time it cycles. What I think is happening, sometimes when you fire a round, the gun extracts, ejects and picks up a fresh round BUT the hammer follows the bolt home... DEAD trigger... rack... reset.. BANG.

Separate the upper from the lower. If the hammer is cocked, place your thumb on the hammer to keep it from smacking the back side of the magwell and pull the trigger releasing the hammer. Now pull and HOLD the trigger to the rear, try cocking the hammer and see if the disconnector/ secondary sear grabs the hammer. If it does, slowly release the trigger, the hammer should transfer to the primary sear on the trigger. repeat this a bunch of times, wiggle the hammer in each position and even the trigger BUT make sure you don't allow the hammer to smack the magwell! I'm thinking at some point during the testing you'll find the hammer is NOT retained by either the disconnector(most likely) or primary sear.

If you do find that the disconnector is the problem, the first thing to check is that the disconnector spring is installed correctly, one end will have a larger in diameter coil, this large coil is pressed into the trigger

Not using an extended buffer, just a standard carbine. It sounds like it is short stroking. However, I am not having any FTE's. Would this make sense? I will check out the ASC follower tuning.

Just read this, will try and report back today.Thank you.

As I stated earlier and by Shultz.

It turns out the GS2 hammer pin had walked out and that was leaving the hammer leaning to the side. I believe this will be the culprit. I'm an idiot for losing the anti-walk "c" and forgetting to replace it. Would this make sense as the source of my issues (at least my AR 9 pistol related ones )?

Yes - Pin out on one side would cause the symptoms...  Since they are Gieselle kit pins I believe they don't actually use the spring to hold them in, just the c-clip.  I guess you are off to the hardware store for a C/E clip.

I'm curious how the hammer pin walked on your G2S.  Did the hammer pin retainer slip off?

He says he lost the clip. Hope that fixes it up OP.

Missed that...  Thanks.

So now am I am trying to get the muzzle device off and can't get the darn thing to budge. I took it to my gun shop and they tried as well, no dice. I took it home, heated it up with the heat gun, still nothing. The main problem is that the flat spot on it is under the rail. So I guess I have to remove the rail to use a wrench? I have been trying to use a screw driver in the slots. Any suggestions? My search of the site came back empty.

Take off the rail.  Secure the barrel AT THE REAR OF THE MUZZLE DEVICE in a sturdy barrel vise.  Use a standard wrench (A1/A2 flash hiders have 3/4" flats) to unscrew the muzzle device.  It screws on/off from the muzzle end - don't try to take it off backward.

Still haven't gotten my armorers wrench to take the hand guard off, but I had the pistol out at my friend farm this weekend and it ran like a dream. Love this thing. Will report back when it's got a can on it.

ETA: thank you guys for your help.

I noticed you were using UMC ammo.   My 9mm AR hates the UMC stuff, it would get random failures to reset the trigger.  Changed ammo to Winchester white box and problem was cured.   Also with the UMC I would from time to time get rounds that just didn't sound right.