First off, a giant mea culpa on this review. Specifically, the lateness thereof. The good folks at CMMG sent me the Resolute 300 series upper receiver in 9 mm, along with a pile of magazines and magazine inserts, not quite a year ago. I had originally planned a really slick video showing how quickly and easily the Resolute upper, combined with the AR-15-based 9 mm magazines, allowed you to swap effortlessly between a 5.56 NATO-chambered rifle and a pistol-caliber carbine. The perfect being the enemy of the good, I still haven't managed to get the video done. Rest assured, though, the process is ridiculously easy. Seriously. If you can separate an upper receiver from a lower receiver on an AR-15, you can swap back and forth between 9 mm and 5.56 NATO.
And that's what piqued my interest in the CMMG setup, to be honest. Running an AR-15-based pistol-caliber carbine has historically required either a cumbersome magazine well insert or a custom-designed lower receiver. That's just how it was: you either used Colt 9 mm mags or Uzi mags, the metal, straight-stick variants, and you dealt with some sort of block in the standard AR-15 lower receiver, or you could use Glock magazines in a not-quite-conventional lower receiver. For the first time, you can use a magazine identical in feel and design to the 5.56 NATO variant the standard AR-15 has used for decades. Thirty rounds fit in the magazines supplied by CMMG, and we were able to actually get 30 rounds in each magazine. The inserts function equally well, allowing full-capacity loading in any 5.56 NATO 30-round PMag body.
With the radial-delayed blowback 9 mm upper, feeding, firing and ejecting rounds worked without hindrance. Loading the magazines is simple--it's the same straight, downward push as with .223 Rem. You do have a limited area in which to load the rounds, simply because a standard 5.56 NATO magazine is significantly longer than a 9 mm cartridge, but it's not that hard to figure out. Magazines accepted a full 30 rounds each, both in the complete mags from CMMG and using the inserts in an older PMag I had available. Really, there's not much to say other than the mags are easier to load than Glock mags, the carbine functioned without incident and making the transition is dirt-simple.
So, then, the real question is why would you want this system instead of a dedicated 9 mm rig? And, that's a great question. The 9 mm upper we received for testing is the Resolute 300, which retails for $979.95 and comes with three 9 mm magazines. Other variants are available for less money, with the Resolute 100 series and a single 30-round magazine retailing for $809.95. Most AR-15-based 9 mm carbines are significantly more than that, and the lower receiver really doesn't add a terribly large amount to the cost. Also, and for me this was a big one, if you have an AR-15 lower receiver that's kitted out to your liking, you just drop the 9 mm CMMG upper on it and go to town. I chose the lower receiver from the carbine build I put together a few years back that has a sweet Timney Targa2 two-stage trigger, Mission First Tactical Minimalist stock and grip in addition to upgraded controls from Seekins Precision. I've put a lot of rounds through this lower and it's just plain comfortable.
The CMMG Resolute 300 upper digested the following rounds without incident:
For about $800, I can drop a CMMG 9 mm upper on this lower and train with a centerfire round on most indoor ranges, with steel at defensive distances or just put 100 rounds through my carbine without spending a fortune. Sure, you're not likely to recoup the cost of the upper in the difference between 9 mm and 5.56 NATO, but there are plenty of reasons to opt for the pistol caliber over the rifle. One such instance that might not come to mind immediately is bringing a new shooter to the range. While the kick of 5.56 NATO isn't unbearable, for someone unaccustomed to it, especially indoors, the muzzle flash and concussive blast can be distracting. With 9 mm? Not a problem at all. It's a great step up from something like a Ruger 10/22 when introducing a neophyte to rifle shooting.
And, probably the most important thing of all? It's just plain fun. When all is said and done, at the end of the day, one of the biggest draws of the pistol-caliber carbine is that it's like shooting on "Easy" mode. Little recoil, easy to keep on target and not a lot of money if you're running a couple boxes of ammo through it. With CMMG's 9 mm upper, you can even keep your favorite AR-15 lower receiver. There's a lot to like about this system.