Pat’s Product Review: CMMG Mk9 T 9mm M4 Carbine

Every once in a while, I’m fortunate, in that, I get a gun sample, that no one else has, and when that happens, I jump on testing it, and follow-up with an article. Such is the case with the new CMMG Mk9 T – and you won’t find this one on their web site yet, it’s not due to be released until the SHOT Show at the end of January 2014.

I’ve owned a couple of the CMMG M4gery carbines over the years, and I still have one, and found them to be great shooters, and priced below what other similar set-ups would cost. I’ve also owned a couple 9mm M4gery carbines over the years, and some were good, some were ok, and some were just bad! I always thought, owning a 9mm handgun, and a rifle or carbine chambered in 9mm made good sense. Back in the old west, many gun owners owned a 44-40 revolver and a 44-40 lever-action rifle or carbine – just made good sense to have guns chambered in the same caliber back then, and it still makes sense today.

A quick run down on the specs of the Mk9 T is in order. First of all, it appears to be very similar to most other AR-type M4 carbines, at first glance. However, as already mentioned, it is chambered in 9mm, it comes with a 16″ medium weight barrel, and has an A2 flash suppressor on the end of the barrel – not that it’s needed, but it completes “the” look. The upper and lower receivers are forged 7075 T-6 aluminum, and the trigger is mil-spec single stage. The gun weighs in a 6.3-lbs and is 32-inches long, with the 6-position telescoping stock collapsed. The barrel twist is 1:10 to help stabilize the 9mm rounds.

The lower receiver is a dedicated 9mm type, in that, it isn’t modified to take a Colt-style 32-rd 9mm magazine…it will ONLY take the 9mm magazine – there is NO adapter inside the magazine well, like so many other 9mm M4 carbines have. The upper has been modified a bit, in that, the ejection port door has been trimmed and shortened, and there is an added brass deflector, in front of the standard one you’ll find on 5.56mm models. 

When you break-open the Mk9 T and pull the bolt out for cleaning and lube, you will readily notice that the bolt carrier is quite a bit different than you find on the standard M4 – the bolt carrier doesn’t have a separate bolt – instead, the bolt has a recessed face, with an extractor in it, and the firing pin…and the back of the bolt carrier (which really isn’t a bolt “carrier” on this gun) has added weights installed in the back of it – this gun is a straight blow-back operation, not gas recoil operated like many M4s are. The barrel doesn’t have locking recesses in it, either…the rounds feed right into the gun’s chamber.

Okay, so far, we have a pretty basic 9mm M4 on our hands, right? Well, not so fast! The Mk9 T has the KeyMod hand guard, in place of a standard two-piece poly hand guard that is found on most M4s.The Key Mod, is a one-piece affair, that attaches via two hex screws, and the barrel is free-floated, for the most accuracy you can wring out of the gun. The top of the KeyMod has the standard Picatinny-style rail for mounting optics or sighs – the Mk9 T doesn’t comes with any sights at all – a minor complaint in my book. The 3, 6 and 9 o’clock positions on the Key Mod have specially designed holes, for mounting other Picatinny-type rails, that can be had from CMMG – should you choose to add something on the KeyMod. The only thing I plan on adding is an adapter so I can put a forward sling swivel on the Mk9 T. I have the accessories coming, however, in the meantime, I modified an old 6-inch Picatinny-style rail, and bolted it to the KeyMod hand guard at the 6 o’clock position – it works!

When I first received the Mk9 T sample, I loaded-up two 32-rd magazines – the gun comes with one magazine, however, I requested a couple spare mags. I had a mix of Tulammo 9mm in FMJ and Black Hills 9mm, some of their 100-grain frangible TN ammo. To be sure, the magazines are a bear to load the first few times, the spring is stout – fully load your magazines and let them sit for a few days, to take a set, and after that, they are much easier to load by hand. Most of the time, during this quick function test, the Black Hills 9mm ammo functioned – but every once in a while, I had a stove pipe malfunction, with a round coming out of the magazine about halfway and sticking straight up – causing a stoppage. With the Tulammo, I had untold malfunctions, with the rounds coming halfway out of the magazine, and once again, sticking straight up.

CMMG, like many AR makers specifically states to not use steel cased ammo in any of their guns. If you do, you will void the warranty. Now, a word on Russian-made, steel cased 9mm ammo. I have found that, when loading this steel cased ammo in many double-stack 9mm magazines, that there are similar malfunctions. What is happening is, the rounds don’t rotate inside the magazine, as the rounds move upwards…they tend to “stick” a little bit, and it doesn’t matter if the cases are poly coated or lacquer coated, the just don’t feed smoothing up the magazine tube. So, I attribute the problems with the stove pipe malfunctions to the steel cased ammo. The Black Hills 100-grain frangible truncated ammo – most of the time it fed just fine. However, the few malfunctions I had, I believe are due to the rough bullet surface, they are compressed powdered copper – not nice and smooth bullets, and some guns won’t function all the time with this bullet shape or bullets made out of compressed powdered copper.

Before I could do any accuracy testing, I had to wait on some folding sights for the Mk9 T. I ordered a set of polymer sights from CDNN Sports  and even though they are only $29.99 – I’ve used them before, I’ve found them to be pretty rugged for the price. I attempted to mount a brand-new BSA red dot sight on the upper receiver of the Mk9 T, however, right out of the package, the scope was broken – not good. So, I was forced to wait several days for the USPS to deliver the sights from CDNN Sports, and the package was several days late in coming.

In the meantime, I took the Mk9 T out several more times, with different ammo, and had absolutely no malfunctions at all. While some might think the Mk9 T is picky about the ammo it will shoot, it is not! I’ve had problems with Russian-made steel cased ammo in a lot of double stack magazines. And, the Black Hills 100-grain frangible truncated ammo – again, I’ve had problems with this bullet design and material – for the most part, this Black Hills ammo has run in all my other 9mm handguns, it just wasn’t quite 100% reliable in the Mk9 T – it was probably 95% reliable.

When my sights arrived from CDNN Sports, I got out there and zeroed them, using Winchester USA brand white box 115-grain FMJ ammo – at 25-yards. During my accuracy testing, I had miserable weather, high winds, and heavy fog. I used Black Hills 115-grain FMJ, Buffalo Bore 147-grain JHP subsonic ammo, and their 147-grain FMJ FN subsonic ammo, plus their 115-grain Barnes all-copper hollow point, TAC-XP +P+ ammo, 124-grain Penetrator FMJ FN +P+, 115-grain JHP +P and their 124-grain JHP +P+ ammo.

The Mk9 T really ran smoothly with +P and +P+ ammo – you could just feel that the gun operated a little bit smoother for some reason, then again, many 9mm carbines thrive on +P and +P+ ammo. At 25-yards, most of my groups were coming in right at 1.50 and 1.75 inches – really great accuracy, but I knew the gun could do better – but with lousy weather conditions, it was hard to keep the gun on target, and my target box kept falling over after every couple of shots, too. Frustrating! However, there wasn’t a lot of time for testing, as SurvivalBlog wanted to get this article and report, on this new model out to our readers as soon as possible.

Was there a winner in the accuracy department? You bet, but it was a tie! Surprisingly, the Winchester USA white box range load came in at 1.25-inches at 25-yards – I shoot a lot of Winchester white box ammo in my articles, and I usually use it for function testing. As did the Black Hills 115-grain FMJ and the Buffalo Bore 124-grain JHP +P+ load. Honestly, in all my shooting, and all the different types and brands of ammo I shot, they were all accurate. However, I believe, given better weather conditions, the Mk9 T will shoot 1-inch groups with ammo is really loves. Still, how do you argue with groups in the 1.25 – 1.75 inch range, in lousy weather?

My one minor complaint, that I already addressed is that, the gun comes with no sights – and it is an easy fix. I installed the inexpensive polymer sights from CDNN Sports, and they work great, and were easy to adjust for windage and elevation. I’d like to see CMMG include these sights with the gun…not all of want to install optics – and to be sure, I don’t see any need to install a magnifying scope on a 9mm carbine. A red dot sight? Yes! Still, it’s nice to have back-up, fold-down/pop-up sights on the gun, for when (not if) your red dot sight quits on you, and the poly sights from CDNN will serve you well. I like the Troy sights for an ARs, but they are spendy, and maybe one day, I’ll install a set on this Mk9 T – but for the time being, I’m content with the poly sights. They fold down easily and pop-up at the push of a lever.

The Mk9 T has a full-retail price of $1.149.95 – not cheap, to be sure. However, I’ve seen dedicated 9mm uppers costing $700.00 – $900.00 alone…and you are getting a complete AR carbine in 9mm, that is dedicated to the 9mm round – not adapted. And, like most CMMG products, you can usually find them discounted a bit – and now is the time to purchase one, while prices on ARs are down once again. Don’t wait for the next mass shooting, because prices will go up, and the guns will, once again, be in very short supply. I love the Mk9 T – it’s fun to shoot, and no recoil to speak of, and all things considered, 9mm is cheaper to shoot than .223 is…and it would make a great house gun – to keep next to your bed.

I’m impressed with the Mk9 T so much so, that I’m going to keep the sample and pay for it after the testing period. I don’t buy a lot of firearms these days, I have all I “need” – not all I “want” – so it’s rare for me to want to purchase a gun sample after testing. But I fell in love with this little 9mm from CMMG, and I like the KeyMod free-floating hand guard, too. Don’t discount a 9mm AR, until you’ve tried one.  – SurvivalBlog Field Gear Editor Pat Cascio