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Old September 16, 2017, 05:41 PM   #1
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9mm ar-15 conversion

I am thinking about getting a 9mm upper for my Colt 6920.

My one concern is about gas in the face. I shoot this rifle left handed with no problems, but I shot the Kel-Tec folding 9mm rifle and that was the first rifle I ever experienced gas in the face where it was a problem.

What I dont know is if it is inherent to the 9mm cartridge being relatively dirty, or design of the 2000 (I am assuming a blow back vs. DI system?).

The other option would be to save for a CZ Scorpion EVO 3 S1 Carbine but that still leaves the same question about gas in the face.

Any one have any experience with doing this?

Thank you
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Old September 16, 2017, 10:20 PM   #2
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I have an AR in 9mm and Ive shot the scorpion carbine. The AR in 9mm is not at all gassy for me. Straight blow back seems to push it all down the barrel.

The Scorpion is the biggest let down Ive shot in a long time. High points have better triggers.

My advise is go with the 9mm upper. That being said you'll probably want to keep a heavier buffer around to swap out when you put the 9mm upper on. Also of you are gonna be putting a mag block in your AR you can only run straight mags, IE Colt/UZI mags, not that thats a bad thing.
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Old September 17, 2017, 10:04 AM   #3
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Use a gas deflector in the ejection port. The higher cost 9mm uppers all use them. I don't shoot lefty, so I don't know how effective they are...
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Old September 19, 2017, 03:15 PM   #4
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"Bark and Blast" from a blowback firearm ( PCC )will always be different compared to a locked breach like a AR15.

You can help minimize the "BnB" by using stiffer springs and heavier buffer weights.

My 9MM Colt pattern PCC is using a .308 Tubbs Flatwire Spring and a 9MM extended 7.5oz buffer.... it runs like a champ.... very smooth.

You can also cut down on the smoky aspect by using clean burning 9MM ammo.

Info on spring weights...

Here are some spring weight measurements from David Tubbs... just food for thought.

A2 (old) - 7.6lb closed, 14.5lb open
A2 (new) - 9.1lb closed, 16.7lb open
Tubb Flatwire - 10.5lb closed, 16.3lb open
Tubb .308 Flatwire - 13lb closed, 16.7lb open
SpringCo White ? (standard) - 8.3lb closed, 16.3lb open
SpringCo Red (extra) - 10.5lb closed, 18.4lb open
SpringCo Orange (increased) - 13.9lb closed, 24.8lb open

IMHO, an extra power spring isn't needed , nor wanted.

I tried both the "Red" and "Orange" ... both caused needless muzzle dip, and were slamming the BCG really hard into the breach face
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Old September 19, 2017, 03:51 PM   #5
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When I put together a 9mm upper I went with an Oly upper that used Sten mags to feed it. I love it and it works great, haven't had any problems but am not a lefty.
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Old September 21, 2017, 10:13 AM   #6
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The other option would be to save for a CZ Scorpion EVO 3 S1 Carbine but that still leaves the same question about gas in the face.
You are not going to get gas in the face with a Scorpion Evo. The ejection port is fairly far forward and that's the only channel for gas to escape.
The Scorpion is the biggest let down Ive shot in a long time. High points have better triggers.
The stock 10lbs+ trigger does stink, but can be improved vastly with a $9 HBI spring kit and a few minutes. Mine dropped to just under 5lbs with just the springs.

Thinking about doing a dedicated 9mm AR and there are so many, so many options.
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Old September 21, 2017, 11:45 AM   #7
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"...this rifle left handed..." More likely because it was a Kel-Tech than anything else. They're not exactly high end kit. A 9mm AR isn't a Kel-Tech. Shot a Colt M-16K years ago with no fuss. Except the Colt rep wouldn't let me keep it.
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Old September 22, 2017, 09:16 AM   #8
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I have an AR9 and I also have the folding Keltec sub 2000. I'm right handed but I can see how the Keltec would put gas to a lefty's face. The sights are short so you really have to get your face down on the stock tube right behind the ejection port. My AR9 isn't like that.
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