9MM AR-15 style Carbine Build Documented

[clandestine]

I forgot some more history to help in the detective work...

Since Brownells was taking so long to deliver, I test fired the gun
without a bolt catch. So it had the new ST-9X buffer and new flat
wire spring installed. Shot about 100 rounds and it never misfired
at all. So it just makes me think that the new bolt catch is somehow
interfering with the trigger or the bolt.
-Kramer

It certainly sounds like there is something going on in the bolt catch assembly. This may be something the manufacturer should replace (the lower), or shoot it without a bolt catch some more to further troubleshoot.

 

[Boss94]

I finally got a new bolt catch and associated hardware from brownells.
Boy, their shipping is slow. Reminds me of pre-Amazon days. Get so
spoiled by the prime shipping. But I digress.

I went to install the new bolt catch. My lower receiver has a set
screw instead of a pin holding the bolt catch in. I went to take
the pin out with my hex driver and NOOOOOO, the hex driver started
spinning and rounded out the hex head. Screw did not budge.
I may have put some blue loctite on it when I assembled it--not
sure, but if I did that was a big mistake. I tried all kinds of
things to get it out. Heated it up with torch and soldering iron.
Tried epoxing my driver into the head. Filed the pin part to
a half round and used a pliers to twist it. All to no avail.
Finally I went for the nuclear option--drilled it out. Which
was hard because a drill will hit the receiver surface normally.
Used lots of extensions on a drill to do it. It worked!!!
Got the pin out. At this point I had to use a roll pin to put
a new catch on. I drilled out the front part of the mount for this.

The new catch I got seemed beefier than the last one. It also has
an extended lower part so it is easier to depress manually to catch
the bolt. Metal also looks harder. I used a shim washer on the rear
side of the catch to try to take out play in its fit. Here is
the new catch installed:




You will note I damaged the black finish while drilling out the
old set screw. Anyone know of an easy thing to use to touch up
that black anodized like finish?

The bolt catch worked with the new ST-9X buffer that I had installed
earlier. There is about 1mm between the buffer and bolt hitting
back of buffer tube and going forward to bolt catch--very tight
spacing. Had me a little worried.

Got to take the gun to the range today for test firing of new
configuration. Loaded 10 rounds. Fired two and then, ARGHH, could
not pull trigger to fire third round. Pulled charging handle
and an unfired round ejected. Aim, fire, and it shot. Of next
six round 2 more misfire in the same way

Loaded 15 for next set and it only misfired once which made me
think that maybe it just needed some magical "breaking in".

But in the end, it still seems to be a problem. Put 150 rounds through it
in total. The last set I loaded a full 32 rounds and it misfired on 4 of
them. All the misfires it was just that the trigger would not pull.
What could cause this? At first I thought it was that the trigger was not
getting cocked. But when I got home I played with it and it seems the
trigger cocks once the bolt moves only an inch rearward. It seemed a
round was feeding fine because it always ejected one after the misfire.
The bolt looked to be visually closed all the way. Every time it
misfired, I would pull the bolt back with the handle all the way
and the next round fired fine. I did not open up the rifle to see
the state of the trigger after the misfire--not sure I could even
do that safely. Is it that the trigger is not cocked, or is it blocked
maybe? Any ideas? Before the new buffer, spring and bolt catch it
did not misfire once with 300 rounds fired. So something I did has
messed things up. Here is a photo of the relevant parts as of now
(note it was clean before I did my test firing; click on the image to get a hi-res version):

In your last picture it looks like the pin holding your weight in the bolt carrier is backing out. Maybe it's a optical alusion? ?

 

[clandestine]

In your last picture it looks like the pin holding your weight in the bolt carrier is backing out. Maybe it's a optical alusion? ?

It is.

Great eye!

That can cause cycling issues.

 

[Boss94]

Thanks !!That would explain why the hammer wasn't always getting cocked. Just machined 1 of those bolts to use on a troy stock. I have seen some of those bolts with the wrong firing pin springs in them .

 

[Kramer]

Thanks !!That would explain why the hammer wasn't always getting cocked. Just machined 1 of those bolts to use on a troy stock. I have seen some of those bolts with the wrong firing pin springs in them .

Do you guys mean the thing marked by my yellow arrow?




If so, how do I go about securing it? The 9mm CMMG bolt is a bit of a mystery to me. I cannot find much info on the web about its different parts or how to disassemble/assemble it.
-Thanks, Kramer

 

[Boss94]

Yes that should be flush or below the surface!

 

[Boss94]

Use a punch and a hammer and drive it down.

 

[Kramer]

Use a punch and a hammer and drive it down.

OK, thanks, I appreciate your help! Took bolt out and used punch to center that pin in the bolt assembly. It was definitely sticking out the right side. I looked in the upper receiver but did not see any scratches from it. I will try it out this afternoon if possible. Thanks for the help. Obviously that was something that needed fixing. Whether it was causing the mis-fire problem, we will see, but I am hopeful. Here is the bolt after I centered the pin:

 

[Boss94]

Looks much better! !! If it backs out again replace the spring pin as they shouldn't back out.

 

[Kramer]

Fix Range Tested

I took the carbine to the range to test the bolt weight pin fix. I shot 150 rounds. On the 96th round it did not fire. Same deal--trigger would not pull as if it were not cocked. This time I payed a lot of attention to the state of the gun. I removed the magazine (4 rds left). I carefully pulled the bolt back a tiny bit to get a peek at the chamber--there was a round in the chamber and it looked fully seated. I then tried pulling the bolt back incremental amounts and then test firing. Did not fire with 1/2" bolt retract. Nor 3/4", 1". At about 1.5" of retraction it fired. I definitely think the trigger was not cocked.

Shot the remaining rounds and another 50 round box and did not have a problem with any of them. So in the end I had one no-fire in 150 rounds.

When I got home I took things apart again. The bolt weight pin was positioned perfectly as it was before the test fire. So it did not move. I did not see anything that looked out of place. I played around with the trigger and the bolt to see how it could move back an not cock the trigger. I did note that when the trigger is being pulled, it cocks differently from when my finger is not on the trigger. It does not cock as easily when my finger is pulling the trigger actively. The bolt has to travel back a lot farther.

My theory is that when it does not fire, the prior round did not send the bolt back far enough and although it is far enough to feed the next round fine, it does not cock the trigger.

At this point I am planning to shoot some more rounds and see if the problem remains or if perhaps with the spring breaking in it just gets better. If the problem remains I am going to have to play with the spring and/or buffer some more.
-Kramer

 

[Kramer]

Love the Maglula!

My biggest complaint about my 9mm carbine is loading a 32 round colt mag by hand. Boy was that painful and slow. I went online and found this at brownells:
MAGLULA LTD. - COLT SMG MAG LOADER; and this video on midway showing a maglula loader made for this magazine. The video seemed a little hard to believe--he loaded 32 rounds in like 15 seconds. But I bought one:




WOW!!!! It is increadible. Works like magic. Just mount it on top of the mag (it has a catch that matches the catch slot in the mag. Then plop in a round, flip the lever on top, plop in another round, flip the lever, repeat. It is so fast and easy. If anyone has these magazines and does not have one of these, I pity you. It is an amazing device. Only problem is it is really easy to load 32 rounds now and I go through so much more ammo in one hour of range time!
-Kramer

 

[Boss94]

You don't happen to have your disconnector spring in upside-down do you??

 

[clandestine]

It still sounds like the FCG. Blowback firearms are hard on the FCG. They can damage disconnectors with trigger slap and the mass can cause hammer follow.

Its your call but the factory FCG will likely solve your problem here.

 

[Kramer]

It still sounds like the FCG. Blowback firearms are hard on the FCG. They can damage disconnectors with trigger slap and the mass can cause hammer follow.

Its your call but the factory FCG will likely solve your problem here.

I am afraid I am a bit of a newbie. Would you mind explaining exactly what could be going on in my case regarding the FCG? What is trigger slap, and what mass can cause hammer follow. Sorry for the dumb questions but hoping to learn.
-Thanks, Kramer

 

[clandestine]

Kns pins are problematic too. They gall and can allow FCG problems.

 

[clandestine]

I am afraid I am a bit of a newbie. Would you mind explaining exactly what could be going on in my case regarding the FCG? What is trigger slap, and what mass can cause hammer follow. Sorry for the dumb questions but hoping to learn.
-Thanks, Kramer

The bolt moves much faster on a blowback 9mm AR than a standard AR with a locking bolt. When the hammer is cocked by a blowback AR, the Hammer can hit the disconnector too hard and not catch. This means the hammer follows the bolt and no bang when you pull the trigger.

Factory and 2 stage fire control groups have tolerances to deal with this but pack triggers and single stage triggers are often more delicate and can have problems when fouled.

The bolt and bffer is heavier on a 9mm AR as well so when the bolt goes home it can bump the hammer off the disconnector and follow the bolt.

 

[Gambler]

I'll bring you a factory FCG so you can try it out.

 

[outrider58]

Speaking of 9mm ARs, I'm considering a build. I'd like it to be integrally suppressed, hopefully meeting the 16'' barrel length(1 stamp instead of 2). Any recommendations on a suppressed barrel. Is there such a thing?

 

[Gambler]

Yes: http://www.srtarms.com/custom.htm. Though it might be cheaper to have a normal suppressor pinned to a barrel.

 

[Boss94]

Speaking of 9mm ARs, I'm considering a build. I'd like it to be integrally suppressed, hopefully meeting the 16'' barrel length(1 stamp instead of 2). Any recommendations on a suppressed barrel. Is there such a thing?

I'd mimic the mp5sd go with a 9 or 10 inch barrel EDM or carefully mill a .06 if I remember correctly hole in the barrel at about 3.4 to 4 inches down from the chamber end. You should be able to find pictures on line and better details on the mp5sd barrels. We use to do them 20+ years ago. I have forgotten more than I care to :-(