Gun Review: CMMG Mk3 LR-308 Rifle

    Central Missouri Machine Guns, more commonly known as CMMG, started as a print shop in the early aughts before heading down the path to become one of the most well-known AR manufacturers in the industry.  Starting with barrels and uppers exclusively, CMMG now manufactures complete rifles under its own name, and has been doing so since 2005 – nearly ten years.  Each of their rifles is made in the USA and is protected by a lifetime warranty.

    CMMG introduced and developed its .308 Mk3 line over the past few years, and now makes a half-dozen varieties of the Mk3 in varying configurations, ranging from $1,600 to $2,000 MSRP.  The subject of this review is the base model Mk3, with an MSRP of $1,599 (the upper by itself is MSRP’d at $999).  My research indicates that street price has hit as low as $1,100 on some variations of the Mk3 (complete rifle), however.

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    Specifications:

    The Mk3 is an LR-308-pattern rifle.  This is emphasized because some readers may not appreciate that there are several varieties of .308 AR-style rifles, and accordingly, it may help to do some research on the matter before investing in a .308 AR.  User “TaylorWSO” at AR15.com has a good post on the fundamental variations, although the information may be somewhat dated, being a post from 2008.  A Google search for “AR-10 versus LR-308” will fill in the informational gaps and likely provide you with enough material to make an informed decision.

    Back to the rifle at hand, CMMG describes the Mk3 as “The perfect 308 WIN rifle for the tree stand or target range.”  The Mk3 employs an 18″ 416 stainless steel heavy taper barrel with a 1:10 twist, a CMMG RKM15 KeyMod handguard and an A1 length fixed butt stock.  The castle nut is not staked on the collapsible stock versions of the Mk3.  It comes with one Magpul 20 round .308 PMag.  The trigger is a standard, single stage GI-style, and the Mk3 lower is outfitted with a standard A2 pistol grip.

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    The 15-inch CMMG RKM15 free-floating handguard with KeyMod.

    First, a word on KeyMod for those of you who are unfamiliar with this relatively new technology: KeyMod is a proposed replacement for the MIL-STD-1913 interface, sometimes referred to as “Picatinny rail.”  In other words, KeyMod – the product of a VLTOR/Noveske collaboration – was designed to replace the traditional Picatinny rail system that has been found on most AR systems since the late nineties.  KeyMod employs a keyhole-type system on the mounting surface (such as a rail) whereby accessories such as lights and lasers can be mounted without the use of tools if they have a KeyMod nut designed to fit in these “keyholes.”  The primary benefit is reduced weight through eliminating un-utilized Picatinny rail interface (which extrudes from the mounting surfaces), and a more ergonomic surface as it is ‘toothless,’ unlike Picatinny rail.  Dozens of manufacturers, including CMMG, have embraced this open-source technology, and it is taking the gun universe by storm at the moment.  You can find more information on KeyMod here.

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    Observe the reinforced takedown pinholes. This reinforcement is present on both takedown pinholes.

    Fit and finish is very impressive after unboxing.  The test rifle had no visible flaws, and everything was tight and uniformly finished.  The stainless barrel on the Mk3 is a very attractive matte bead-blasted stainless similar to the Noveske offerings.  Compare this to other manufacturers such as BCM, who use polished stainless barrels.  The bolt release “ping pong paddle”, safety, and takedown pins all function crisply.

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    Note the striations on the backside of the ejector block and the lower ledge of the upper receiver.

    The CMMG bolt is machined from 9310 steel, and the carrier from 8620.  The bolt is completed with a hardened extractor of S7 tool steel.  The carrier interior is chome-lined, and the carrier itself appears to be phosphated.  The key is staked.  While the staking is a little light (pictured), it appears to be acceptable.

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    Close-up of the gas key staking.

     

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    Note that the Mk3 has no sights, so you will have to purchase a set of front and rear sights for use with the Mk3.  Also note that the standard for BUIS is still Picatinny, so even though the Mk3 comes outfitted with a KeyMod interface, traditional Picatinny rail runs along the top of this gun, from the rear of the upper receiver to the front of the 15-inch CMMG RKM15 free-floating handguard.  Accordingly, this rifle still accepts your same optics from your standard A3-style AR.  However, where the RKM15 meets the receiver, there’s about an inch of flat surface without Picatinny teeth, as is common with non-monolithic handguards.

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    Close-up of the chamber and feedramp cuts.

    The overall length of this rifle with the A1 stock is 38.5″, and the gun weighs 9.3 pounds unloaded.  To put this into context, a standard 16″ chrome-moly barreled Armalite AR-10 weighs 7.9 pounds, but for the extra accuracy and velocity provided by the 18″ 416 stainless barrel in the CMMG, the price you pay includes additional weight.  Moreover, the 17.1 ounce 6061-T6 extruded aluminum RKM15 handguard certainly contributes to that heft, but the free-floating RKM15 will also provide better accuracy and more mounting options over standard AR-style handguards.

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    But how do these specs come out in the wash?

    General Observations:

    Shooting this rifle, summarized in one sentence: If you’ve shot a high-end, stainless-barreled, GI-triggered AR-15 before, this is the same gun, but with more recoil.

    In other words, there were not a lot of surprises in shooting the Mk3:  Unsurprisingly, the Mk3 was very accurate.  With Prvi Partizan PPU .308 FMJ 145grn, the CMMG punched four out of five rounds in one of my 100m strings into a .9″ hole.  To be fair, the first round was a flyer caused by my error, and even including that round, the whole bunch of five fit into 1.4 inches.  This is even more impressive considering this was fired from prone unsupported and with a zero-magnification EOTech optic.  Had this been fired from a vise or with a telescopic optic, I’m sure the Mk3 would have grouped even tighter.  A search of the typical forums have other users reporting that they obtain sub-MOA groups with match ammo, and groups similar to those I obtained (in the one to one and one half inch range at 100m) with standard ammo similar to what I used.

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    Four of five rounds of Prvi went into a .9″ hole at 100m.

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    Also unsurprisingly, there were no failures of any type, and no nuances or quirks with the rifle – everything worked as expected.  The standard GI-style trigger is what it is – the same single-stage trigger on every other stock AR.  Not to say it was good or bad, but it is definitely what you are used to if you have shot an AR before.

    In all, function of the rifle functioned perfectly during the range session, and accuracy was excellent.

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    Negative Observations: 

    And while the range session was impressive from a mechanical perspective, shooting the Mk3 from prone was physically taxing.  Recall that this rifle comes outfitted with an A1 stock and A2 grip.  While perfectly suitable for the lighter-recoiling 5.56mm AR15, these accessories don’t lend themselves to comfort with the more potent .308 variant.  If I were planning on purchasing an Mk3, these would be the first things I would replace, and represent my only (and therefore slight) reservations about this rifle.

    First, as we all know, the A2 grip, though not a bad grip, is not very comfortable or ergonomic for most shooters.  For your average-to-larger shooters, the slim trigger reach can lead to too much trigger on the finger and a sharper recoil impulse than from an improved grip with a little more material in the tang, such as a Magpul M.O.E.  Fortunately, this is a cheap and simple replacement.

    Similarly, the A1 stock can be brutal from prone.  At the risk of sounding fragile, when my range session concluded, I had a bruise where my right pectoral met my shoulder.  And I am no stranger to .308 rifles, but this was my first outing with an A1-equipped .308 AR.  (See the embedded video below and note that the recoil would gradually tilt my tightly-secured GoPro upwards after each successive shot.)

    Range Session: CMMG Mk3 .308 AR from Pelican Handgun Instruction, LLC on Vimeo.

    On a scale of FAL (more pleasant) to HK91 (brutal), I’d say that the experience with the A1 stock on this rifle was closer to shooting the HK91 with a collapsing stock (the HK91 collapsing stock is unceremoniously known as the “cheese grater” in HK circles).  But again, this is a cheap fix, and I think a rubber buttpad instead of the hard, plastic A1 trapdoor plate would do the trick – at least it seems to work for the FAL.  Most of you will probably drop a Magpul in place of the A1, anyways, so this may very well be moot.

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    Conclusion:

    In conclusion, the Mk3 is an excellent and well-made .308 rifle, apparently capable of sub-MOA accuracy with inexpensive factory brass-cased ammo.  And while my chief complaints relate to the recoil-management capabilities (or lack thereof) of the A2 grip and A1 stock, in the end, I would say it is better to have generic and easy to replace components such as the grip and stock to keep the price low, rather than to mark the cost up and sell a rifle to the user with a more expensive configuration that may not be suited to that particular buyer.  Basically, the buyer who drops $1,000-$2,000 on this rifle isn’t going to be daunted by an extra $100-$300 to customize those aspects of the rifle, so I would agree with CMMG’s approach in leaving the ultimate configuration up to the end user.  However, it might not be a bad idea to spend a couple of bucks to include a rubber pad for the buttstock for those shooters who have no plans to further invest in their Mk3.

    Related to dollars and cents: As I said, my research indicates that this $2,000 MSRP rifle is anywhere from the $1,100 to $1,500, street price, and I quickly found a few on Gunbroker in the $1,200 range, NIB, however, I note that the Mk3s I have seen on the low end of the price spectrum appear to be older models without the KeyMod RKM15 handguard.  That said, if the quality of the older models of the Mk3 match the build and performance of the model I evaluated, I would say that $1,100-$1,250 for this rifle would be a steal.  In conclusion, if you are in the market for a .308 AR that has more amenities than your standard Armalite AR-10 or DPMS LR-308, you would be well-served to evaluate to the Mk3 for yourself.

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    James Reeves

    • NRA-licensed concealed weapons instructor, 2012-present
    Maxim Magazine’s MAXIMum Warrior, 2011
    • TFBTV Executive Producer
    • Former Regional Sales Rep, Interstate Arms Corp., MA
    • Champion, Key West Cinco De Mayo Taco Eating Competition
    • GLOCK® Certified Pistol Operator, 2017-2022
    • Lawyer
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