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Posted: 3/21/2026 4:57:49 PM EDT
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What is stopping me from using a "regular" ar15 upper receiver to build an ar9mm? Besides having a larger than needed ejection port, is there any other issue? |
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They are the same upper, the 9mm one doesn't have a hole for a gas tube but you can still use a regular AR upper. The ejection port thing was just a different port door and has since been proven not needed and a possible source of issues. Read more here https://blowback9.wordpress.com/2022/01/22/the-9mm-ar-a-flawed-design/ And specifically here https://blowback9.wordpress.com/2021/04/02/everything-about-the-ar9/ And this part covers the ejection port door/brass deflector https://blowback9.wordpress.com/2021/08/05/stop-recommending-obsolete-parts-obsolete-9mm-ar-partsfeatures-that-people-still-recommend/ |
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Originally Posted By buzybraza: What is stopping me from using a "regular" ar15 upper receiver to build an ar9mm? Besides having a larger than needed ejection port, is there any other issue? I don't understand the other poster's comment regarding "Receiver Extension"??? The buffer tube is on the lower receiver and is kinda needed to hold the spring and buffer... The comment regarding Colt Mags being too wide makes no sense. Colt Mag inserts for lowers actually take up some of the EXTRA width in the magwell. |
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Originally Posted By MaxSpeed74: Nothing is stopping you. It will work fine. I don't understand the other poster's comment regarding "Receiver Extension"??? The buffer tube is on the lower receiver and is kinda needed to hold the spring and buffer... The comment regarding Colt Mags being too wide makes no sense. Colt Mag inserts for lowers actually take up some of the EXTRA width in the magwell. Originally Posted By MaxSpeed74: Originally Posted By buzybraza: What is stopping me from using a "regular" ar15 upper receiver to build an ar9mm? Besides having a larger than needed ejection port, is there any other issue? I don't understand the other poster's comment regarding "Receiver Extension"??? The buffer tube is on the lower receiver and is kinda needed to hold the spring and buffer... The comment regarding Colt Mags being too wide makes no sense. Colt Mag inserts for lowers actually take up some of the EXTRA width in the magwell. Also curious about that. I'm not sure what he's talking about, but he's built more 9mm uppers than just about anyone, so... |
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The junction of the 1” bore and the radius in the upper that acts as a 5.56 magazine stop is what I’m talking about. When we first did the 635 uppers at NoDak, one of the OG retroheads sent me an Original Colt 9mm upper so we could copy it. The 3 features that were different: No cam pin cut No gas tube features An additional skim cut in the upper to allow the 9mm magazine to penetrate higher past the stop radius |
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Originally Posted By mike_nds: The junction of the 1" bore and the radius in the upper that acts as a 5.56 magazine stop is what I'm talking about. When we first did the 635 uppers at NoDak, one of the OG retroheads sent me an Original Colt 9mm upper so we could copy it. The 3 features that were different: No cam pin cut No gas tube features An additional skim cut in the upper to allow the 9mm magazine to penetrate higher past the stop radius |
"Major, with the weapon shops out of the way we can introduce steadying laws that could not be flouted." -A.E. Van Vogt 1951
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I don't know what is meant by the receiver extension comment, but I can confirm that you can use any typical AR upper receiver to build a 9mm AR. The fact that the upper might have M4 feed ramps does not affect the 9mm build. I've assembled a bunch over the years. The forward assist is not necessary, but most uppers have them. Aero Precision used to have an upper with no forward assist, and I tried to use them for my 9mm and 22LR ARs, but its not necessary and is just a cosmetic thing. I also used to install the big brass deflector, but on other builds with no deflector I did not have an issue, so I gave up on using them. Perhaps they are more necessary if you shoot lefty. The standard port door is a good feature to have, I purchased a few port covers that are engraved on both sides with the caliber. That way you have a reminder of the cartridge in that AR. I use them for my 7.62x39, 9mm, and 22LR ARs. I hope your build goes well, my 9mm ARs are my most fun rifle to shoot. |
| Have used both patterns but prefer the standard upper. Have also run both pistol specific lowers using Glock mags as well as standard lowers with the Mean arms Pmag inserts and the Sylvan magwell adapter without issues. I used a side charging upper from JoeBob with a Glock mag lower to build my 380 rig as well. |
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I use a standard AR upper with a standard dust cover that has the caliber lazer etched for quick identification. The lower is a standard AR lower that I’m using a Sylvan adapter with. I have both the Glock and the Beretta adapters. The Glock adapter has run flawlessly. The Beretta not so much. I do get FTFs, and I haven’t put enough rounds through it yet to determine if it’s a mag issue, or if I don’t have the adapter adjusted right to the frame. It has a wired stick to be CA legal. Stock is really quite comfortable. https://imgur.com/a/OwXj4RS |
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Originally Posted By buzybraza: What does that mean? Different system than buffer, buffer tube and spring? Originally Posted By buzybraza: Originally Posted By User55645: I prefer to do without a receiver extension. What does that mean? Different system than buffer, buffer tube and spring? 9mm AR barrels don't have a receiver extension screwed on the end of the barrel. They have a solid breech end, with the chamber machined directly into the barrel. This is for blow back operation as there is no need for the lugs found in "regular" barrel extensions. |
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Originally Posted By EDL: 9mm AR barrels don't have a receiver extension screwed on the end of the barrel. They have a solid breech end, with the chamber machined directly into the barrel. This is for blow back operation as there is no need for the lugs found in "regular" barrel extensions. Originally Posted By EDL: Originally Posted By buzybraza: Originally Posted By User55645: I prefer to do without a receiver extension. What does that mean? Different system than buffer, buffer tube and spring? 9mm AR barrels don't have a receiver extension screwed on the end of the barrel. They have a solid breech end, with the chamber machined directly into the barrel. This is for blow back operation as there is no need for the lugs found in "regular" barrel extensions. 5.56mm AR barrels don't have a receiver extension screwed on the end of the barrel either. They have a barrel extension screwed on. AR lower receivers typically have a receiver extension, commonly called a "buffer tube", screwed into the lower receiver. FPDNI. |
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