Hello!
I am building my first AR in 9mm and ran into an issue. Bear with me, this post may run a bit long...

The following Parts are being used:
[list=]
Dedicated Colt Magazine 80% Lower using the 5S Jig system.
Matrix Arms dedicated Upper with Side Charging Handle
8.5" Kaw Valley Precision Barrel
Kaw Valley Precision Flash Compensator
Kaw Valley Precision Handguard 9"
Kaw Valley Precision BCG
Kaw Valley Precision 7.5oz Buffer and Heavy Buffer Spring
Shockwave Blade and Tube.
[/list]

The lower has been machined correctly. All measurements are correct. The 5S Jig is really simple to use and it works flawlessly. 60min and you are done.
The assembly went smoothly but I noticed one thing:The ejector in the lower has some play after mounting it.

Now to the problems:
First run to the range and the first round went of well.
Rounds 5-7 did not go well due to FTE issues. The ejected round kept getting stuck ontop the new round.
Not a good start.
The idea then was to use a heavier buffer to slow the BCG down. I have 10.5oz buffer from Kaw Valley Precision and used it.
Things really went south after this... multiple stovepipes, FTE and two blown out cases...
I cleaned the parts and tried it again.
Heavier spring and heavier buffer- no success. Light Spring (H1 from an AR-15) and lighter buffer (7.5oz)- bumpfire issues and FTE. Not good.
Right now it is setup with the heavier spring and the 7.5oz buffer. Not really eager to test it...

Question:
Does anyone have experiences like me with such a setup. Can anyone point me in the right direction on how to solve this?
I would really appreciate any input you guys can give me.

Thank you!
Matt

Is the ejector working properly? If it's not, the spent case will come backwards but not get thrown clear of the gun.

@MadPick:
I tested the ejector on the BCG and the mounted ejector with a spent case on my bench. It seems to work fine.
Now, this is in slow motion and it seems to perform correctly. At least I believe so.

My concern is the ejector in the lower. How can I mount it solid. Should it move?
Anyone know of a gunsmith or experienced builder North of Seattle that can help or advise?

Thank you!

Have you tried a buffer spacer?

https://www.joeboboutfitters.com/New_Fr ... spacer.htm

@DoNotBugMe:
I called Kaw Valley and they strongly advise against it.
Their buffer is a bit longer. It should therefore compensate for the spacer.
I wanted to go with $1.50 in quarters initially...

The part on the BCG that you're talking about is the extractor; this is the little claw that fits over the rim of the case, and holds the case as the BCG travels backwards. Then, once the bolt is most of the way back, the left side of the case hits the ejector (which is mounted to the lower), and this knocks the empty case to the right and out of the ejection port.

Many ejectors have some wiggle to them, so that in itself isn't necessarily a problem as long as the case hits the ejector every time.

I've had to adjust the ejector on one of my 9mm ARs. I don't know if yours is mounted in a way that allows any adjustment or not.

You mentioned two "blown out cases." Can you provide pics of those, or a better description of what happened?

Thank you for clarifying the terminology.
So, the ejector can have some play? If so, then this makes things a bit more complicated. I was hoping for an easy fix...

I cannot post pictures right now, but the two blown out rounds were basically double feeds.
The spent case was in the way of the new case. It got damaged very badly. I had to take the upper apart to remove them. Both spent cases were stuck above the new rounds.

Let me ask this:
A heavy buffer and a 308 carbine spring would slow the bcg down even more compared to a standard spring and a light buffer, or?
I have a 308 spring here and could try it tomorrow...

Disclaimer: I have zero experience with 9mm ARs other than I've shot a few.

I'm noticing that the ejection port is rather short (front to back) on the Matrix upper compared to the uppers that Kaw Valley makes and other AR9s that I've seen.

I checked out the Matrix lowers on their website and it looks like the ejector is longer. Might just be the way the photo was taken or that it's a glock mag lower (again, zero experience).

Have you seen the matrix upper work with the Kaw Valley parts and the colt lower? I'm thinking the kaw valley parts are trying to eject the case further back than the matrix upper will allow for and the case is bouncing back into the receiver.

Oh . . . so by "blown out" you mean that the cases were smashed up from the bolt hitting them, but the rounds didn't actually fire? It's not that they fired out of battery or anything like that, just that they got mashed?

Yes Sir!
they did not fire out of battery. The got smashed so far up into the upper receiver, I had to take the thing apart and use a plied to get them out. I will upload pictures tomorrow.
At first I thought it was a hot round since I used my reloads.

As to the other comment in regards to the port opening. Yes, you are right- it appears smaller than a normal AR port. It actually is smaller too.
Kaw Valley uses a standard upper without forward assist. Nothing special there. The upper I chose was recommended for future suppressed use of the 9mm. Since I am a lefty shooter, it was/is important for me to minimize any blowback in my face.
To the best of my knowledge a standard upper cannot be sealed this well since the charging handle is the issue. Gas Buster might reduce it....

I have reached out to Kaw Valley and to Matrix Arms.
Ian at Kaw Valley is very helpful and basically confirmed my fear: he recommends a standard upper for testing.

Matrix Arms disagrees and offered to take a look at my pistol. Meaning I have to ship it to them. This opens a new can of worms, since it is a non serialized pistol... Unless I take it apart and ship it as parts... Crazy!

Any thought or idea is welcome!
Anyone know a trustworthy gunsmith?

Matt

I also don't think the ejector should move. You might need to shim it to get it solid, then bend it to the right spot.


Sent from my iPhone using Tapatalk

Might need an ejector shim, but I'd have to look at it.

As for the blowback when suppressed, it's not really a problem with AR-9s because they don't use a DI gas system. At least it isn't a problem with mine.

See if Matrix can give you a shipping label and then you can drop it off at FedEx immediately. They can also ship it directly to your home when they're done with it.

Thanks for all the help!
As of today the following things happened:
I ordered a new upper from Kaw Valley. Its a standard upper with aft charging handle. I was insured that it would not blow into my face. Lets see.
Matrix Arms want their upper back. I 'll cut my loses and return it to them. As much as I like the side charging handle, I cant afford to tinker with the gun for ages.
I contacted the Manufacturer of my 80% lower. They are not sure why it would have play. Maybe I'll get a second ejector claw, sand it down as a shim and see what happens.
The new upper should be here by Wednesday and if all goes well, I'll be at the range on thursday morning...
I'll keep you posted.

Well I might be a little late here. But....
With the smaller than average ejection port you have a smaller window of opportunity to eject that spent case.
The ejector (the part in the lower ) needs to be bent or "tuned" so it hits the case precisely to launch it out of
The ejection port. This is all trial and error. You will need to "read" your spent cases and ejection port for dings and dents
To determine which direction to bend the ejector.
Second note. The loose ejector should not be a major issue. Both my CMMG lower and Black Creak Precision ejectors are loose.
Also with your type of Last Round Bolt Hold Open. You cannot shim the ejector because the screw that holds the ejector also holds the LRBHO and it Must move freely. Shimming the ejector will bind up the LRBHO

Good luck.