This is a follow up of the post Disappointing AR9 news.

For those coming late to the festivities, I acquired from different sources a 9mm KVP upper and a Spike’s Tactical lower (Glock mag compatible) and they refused to mate properly.

On a rare attack of common sense, I decided to email both Kaw Valley Precision and Spike’s Tactical to get some answer. First of all, both companies were exceedingly fast on replying to my query for which I am happy and encouraged since past experiences with other companies have been utterly disgraceful.

Ian Reel from KVP was upfront and stated that he had not seen many lowers like mine.  I won’t deny I would have been a tad incredulous, but reader @Snarkyposters from X (thankyouthankyou) had already pointed me to several AR9 sources of information and I formed the suspicion that this platform is a cluster flock of seagulls.

And then I got the email from John Dowhy of Spike’s Tactical which gave me the final answer to my question(s):

Hi Miguel,

Our Glock-style 9mm Lower has a LRBHO bracket that runs along the side of the magwell. Our 9mm Uppers have a relief cut (in the picture attached) to accommodate for that bracket. Other brands of Uppers often do not fit with our Lower because of just what you highlighted. The Upper hits the bracket and cannot close.

There is no Milspec for 9mm AR’s so mixing brands can be problematic. We have had customers make a relief cut themselves in other brands of Uppers or abandon the other brand and use our Upper.


Bingo! Let me show again the photo of my upper:


The arrow points to the point of unwanted contact between upper an lower and the upper does not have the relief cut.

Mystery solved. Problem? Not so much.

There is no Milspec for 9mm AR’s so mixing brands can be problematic.

Those are words you need to live by if you want to build your AR9.

Right now, I see myself with two AR9 half builds and nowhere to go unless I decide to sink more money in the project which I am not happy to do. I may go with a regular 5,56 lower and one of those adapters for the magwell, but the good ones cost almost as much as a dedicated lower.

I’ll just hit the pause button for a while till some smarts come to me.

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By Miguel.GFZ

Semi-retired like Vito Corleone before the heart attack. Consiglieri to J.Kb and AWA. I lived in a Gun Control Paradise: It sucked and got people killed. I do believe that Freedom scares the political elites.

12 thoughts on “AR9 issues update. Not easy solution.”
  1. as I mentioned in your previous post, pistol caliber ARs are difficult for this very reason. as there is no “mil spec” to adhere to manufacturers do shiite like this. also they WILL require fine tuning to be reliable..

  2. I’ve been building AR9s for 7-8 years now with a focus on competition and I guess I’ve had remarkable luck, but then again I haven’t ever used a Spike’s lower. I’ve used Matrix, Foxtrot Mike, Quarter Circle 10 & JP. The only brand of those that gave me problems was FM (and their CS burned me. Do not recommend). In most cases, I’ve used regular Aero AR15 upper receivers for these ‘builds’. If you want a quality 9mm lower that has a superbly reliable LRBHO at a good price, look around for the Matrix MA9GL (it can be had currently for $144 from a reseller w/ coupon code). It’s not as sleek as some others, you’ll still have to install a trigger guard & there aren’t any aftermarket competition magwells for it but it has run perfectly with every Glock & Magpul brand Glockazine I’ve used.

    Regarding conversion kits, the Stern is reliable and well made but a bit spendy. [The Stern 9mm bolts are perfect for some of the competition mods, like short stroking, BTW]. Mean Arms makes a magazine conversion device that you insert into a 5.56 PMag shell that changes it to 9mm for use in a standard 5.56 lower (still needs 9mm buffer, of course) but it’s not a serious accessory in my view because the ejector is part of the magazine. “Unload & Show Clear” is a bit dodgy when there’s a live round on the bolt face that won’t eject because the mag was removed. Pardon my ramblings, I hope some of it is useful. If you have any questions let me know.

    1. I’d suggest everyone have a look at the Blowback9 website if you don’t have any idea what #1 is taking about.

      Just Google it.

  3. No, you should be good to just modify the upper, it’s just a regular $50 no forward assist Anderson upper.

    Even if you screw it up, it’s a simple matter of just swapping the barrel to a new one.

    1. I’d chip in for a replacement upper if Miguel’s Dremelation doesn’t go as planned. (Heck, I might have a spare lying around I can just send. I really need to step up the post-move unpacking the workroom.)

      1. Well, shucks. Sorry, I’m at the limit of my knowledge. Best to get a proper lower and sell the one you have. Maybe a local FFL could use it. Good luck!

  4. I have three AR9s I’ve built, but I won’t go with the Glock Mag lowers. I think they look stupid. For one of those builds, it was built off of a Colt parts kit and did get a mag adaptor to take Colt 9mm mags. That is a fun one. One of the others I decided I had enough Magpul magazines and opted for the MEAN ARMS 9mm magazine insert. I didn’t have any issues with those until I stored a loaded mag on my chest rig. The adaptor is made of plastic and caused one edge to bend a bit and doesn’t feed as reliably as it used to.

  5. There is no milspec for 9mm ARs.
    I’d beg to differ there and say Colt pattern is the standard or baseline as everything works off of standard AR15 parts. All other systems, even if better, deviate from that unofficial standard.

Only one rule: Don't be a dick.

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