A 3-Gun Heavy Metal Hammer? CMMG Mk3 3GR .308 Rifle – Full Review

The Mk3 3GR from CMMG is designed from the ground up for the Heavy Metal class in 3-Gun.

To learn more, visit https://www.cmmginc.com/shop/rifle-mk3-3gr-308-win/.

To purchase a CMMG Mk3 rifle on GunsAmerica.com, click this link: https://www.gunsamerica.com/Search.htm?T=CMMG%20MK3&ltid-all=1&as=730&cid=4&ns=0&numberperpage=50&.

Are you a shooter that likes the biggest and baddest guns and chamberings? Well, then the 3-Gun Heavy Metal class is just the thing for you. And, if you are this kind of shooter, then the Mk3 3GR .308 from CMMG was developed just for you.

I refer to the Heavy Metal class as the “He-Man” class, as competitors use real calibers in their shooting. The three guns used in He Man class are .45 ACP handguns, 12-gauge shotguns, and .308 rifles with 20-round magazines (these can vary some, based on division and sanctioning body). The idea of this class is that the use of these heavy calibers requires participants to strike a balance between weight and performance to have a chance at staying competitive. The ranges can be from close quarters to 300-plus yards away. The courses of fire can run up to 40 rounds per stage. Considering this basis for the development of the MK3 3GR, this .308 rifle should be great for serious use.

The muzzle is topped off with a CMMG SV muzzle brake for helping to control recoil and keep the gun on target. Image courtesy of manufacturer.


  • Chambering: .308 Win.
  • Barrel: 18 inches
  • OA Length: 40 inches
  • Weight: 9.4 pounds
  • Stock: Magpul MOE Rifle Stock
  • Sights: None
  • Action: Semi-auto
  • Finish: Matte black
  • Capacity: 20+1
  • MSRP: $1,849.95


As soon as you heft the box up off the porch, you can tell based on the size and weight that it doesn’t hold just another AR-15. The MK3 3GR comes in at 40 inches in overall length, with an 18-inch barrel. Empty, the gun weighs in at a reasonable 9 pounds, 7 ounces. A magazine and 20 rounds of .308 Win. add about 2½ pounds to the package. The pistol grip is a Magpul MOE, which can take different core inserts based on your needs. The stock is the MOE Rifle Stock with a sloping cheek weld which, combined with the shape, allows offhand hold options. The rubber butt-pad locks in tight to your shoulder while sopping up the recoil. The butt-plate is hinged and covers a large storage compartment. The bottom of the stock has bosses you can knock out to allow an accessory rail to be mounted. Slings can be attached with a sling loop, or 1.5-inch QD swivels can be inserted in the front or rear of the stock.

The 3GR is no lightweight, weighing in at 9.4 pounds, but it delivers a lot of power in a reasonably sized package. Image courtesy of manufacturer.

The handguard is an RKM15 free-floated KeyMod unit that covers the low-profile gas block. Image courtesy of manufacturer.

The Mk3 3GR’s upper is mated to an 18-inch 416 stainless steel heavy taper profile barrel, which is bead-blasted to a matte finish. The 5/8 -24 threaded barrel is capped off with a CMMG SV muzzle brake, to manage the recoil of the .308 Winchester. The barrel is ensconced in a RKM15 free-floated KeyMod handguard made from 6061 anodized aluminum. The handguard has a full-length accessory rail for the sights and optic of your choice. Key mod slots are cut down the full length of the handguard.

The lower is billet 6061 T6 aluminum that accepts LR-308/SR-25 pattern magazines. The trigger is a Geissele SSA 2-Stage trigger; the first stage pull is 2.3 pounds and the second stage is 1.2 pounds. The total pull weight is thus 3.5 pounds. The low-mass hammer reduces lock time; this all adds up to a crisp break that is very shootable.

Why A Big-Bore AR?

Considering the legions of AR-15 guns, in all manner of calibers, that seem to launch heavier bullets with each design iteration; where does this gun fit in? In a word: Distance. The .308 is hard-hitting and easy to shoot at extended ranges. We all saw the video a few years ago, where the 300 BLK is hitting targets out to 750 yards. Keep in mind, that was with a 41-foot hold-over! That is not what most of us consider ideal. The .308 only requires 13 ½ feet of hold-over at 750 yards; not an easy shot, but it shows where the .308 Winchester shines. This kind of performance is not free; you must pay for the added weight. This weight has some considerable advantages to offset the cost, as it tames the recoil and hits harder and with more energy. Now all you 300 BLK fans settle down—I like the round, and I think it offers a lot of great things, but it’s not a .308. It’s a compromise round, kind of like the .40 S&W… ugh, that was an ugly comparison… but you get what I mean.

The author topped the rifle off with a Burris Veracity optic and Burris rings. Not the difference in color of the handguard to the receivers.

The scope featured the Ballistic Plex E1 FFP varmint reticle. Image courtesy of manufacturer.

Range Prep (deep dive)

I have realized that I tend to skip over just what I do to prepare a rifle for the range, and what tools I tend to use, so let’s take a deeper dive here than normal. The easy part is choosing an optic to mount up. Mounting an optic can be easy, but there mistakes you can make that can make it a hassle. Also, there are some tools available that can make it even easier.

I selected the Burris Veracity 5-25x50mm scope with the Ballistic Plex E1 FFP Varmint reticle. To mount this to the rifle I used Burris Xtreme Tactical Rings steel rings, as this will be a gun that should have the accuracy to meaningfully employ the high magnification of the scope. The rings will accept both the 30mm tube and the accessory rail of the rifle.

First, I mount the lower part of the rings to the gun. This seems straight-forward, but there are a few details I have found crucial, and the correct tools make these steps easy. The digital Fat Wrench is my favorite tool for this chore, and I’ve tried a lot of the alternatives. The bases all come with torque specifications that they are designed to work within. Too little torque and the scope can move when you shoot, and too much can break the rings. Burris rings and bases have torque specifications ranging from 20 inch-pounds to 65 inch-pounds, based on what you have chosen to use. The digital Fat Wrench takes all the guesswork out, with easy push-button settings combined with both digital and audible readings to precisely torque the fasteners.

The Wheeler Engineering digital Fat Wrench has easy push-button settings combined with both digital and audible readings to precisely torque the fasteners.

Before you torque the rings down, the scope must be level. This assures that the cross hairs are aligned so your windage and elevation adjustments will work correctly. For this task, I rely on the Professional Retical Leveling System, which utilizes a series of bubble levels to ensure that your scope is true to your rifle.

The Magpul MOE Rifle Stock proved to be a nice fit for the author.

Finally, I have my checklist. As most of us (myself included), do not have a range in our backyards, we must load up our gear and head to the range. Nothing, and I mean nothing, can ruin a trip to the range like forgetting one simple thing. It might be a magazine or the correct accessory. You can shoot in the rain and cold, but forgetting a magazine can ruin the whole trip. So, I have developed several handy checklists that I fill in the blanks for based on what I’m shooting, and then check the boxes off as I load the truck.

The author tested the CMMG rifle with a selection loads from Hornady and Sig Sauer.

On the Range

When I got to the range and set up for shooting, I realized that I had not used my check list… and it was time to pay for that. I had forgotten my Lead Sled. Great. No choice but to improvise! I grabbed a case and my camera bag and adapted, but I’m positive that this workaround impacted my accuracy while bench testing the rifle. I had chosen three types of ammunition to test the CMMG with: Sig Sauer 168 gr match, Hornady 168 gr A-Max, and Hornady 125 gr SST Reduced Recoil. Despite my lack of checklist-approved Lead Sled, I was on paper in just a few shots. The accuracy was better than I had expected with all of the ammunition. I always choose different types and weights of bullets, as most rifles have a preference. But even with my makeshift rest, for all practical purposes, it was a dead heat between the three rounds.

The rifle performed well with all the loads tested, hovering around the 1 to 1.5 MOA range.

Functionality was 100 percent, even during what I call my “mixed bag” test. This is where I take partial loads from various boxes of ammunition and mix them all in one magazine, and then check for function. The 3GR ate everything I could feed it. I cleaned out several hundred rounds this way, and I got tired before the gun did.

I was running two different styles of Magpul magazines: 20-round and 25-round windowed. The rifle did not complain with either no matter how I loaded them. The magazine well was easy to charge, with both feeding devices, due to its beveled opening.

The high point of the MK3 3GR was the trigger. This trigger was a pleasure to shoot. It had about the perfect weight for a serious gun and would work on the range, in competition, or in the field, hunting.

The Bottom Line

I like this gun, but I struggle to pinpoint the exact reason I don’t love it. It seems to adequately deliver everything I expect from its class. And I think that’s my sticking point right there: “adequately.” The devil, as they say, is in the details. The upper and lower match, but the handguard is clearly a different shade. The fit and finish have no impact on functionality and have no impact on performance, but this gun is just plain.

The author liked the Burris Veracity scope with the varmint reticle and thought it complemented the CMMG rifle well.

As I compare it to 7.62 AR platforms that I do love, I notice one common denominator: A price tag increase of $500 to $1,000 dollars. I suppose that what I’m longing for are just those bells and whistles that are tacked on, with no performance increase whatsoever. It’s irrational, but I’m willing to pay a little extra for my rifle to stand out. I showed my wife this article, and she commented: “This is why some people buy Apple devices instead of PCs.” But, if you want a rifle that works for a reasonable price, then this one is for you.

To learn more, visit https://www.cmmginc.com/shop/rifle-mk3-3gr-308-win/.

To purchase a CMMG Mk3 rifle on GunsAmerica.com, click this link: https://www.gunsamerica.com/Search.htm?T=CMMG%20MK3&ltid-all=1&as=730&cid=4&ns=0&numberperpage=50&.

{ 13 comments… add one }
  • Jman August 14, 2017, 6:54 am

    Excellent article! Pay no mind to the ignorant trolls comments below. It’s clear by some comments they have never competed or even seen a match because if they did they would realize that the C clamp method of holding is very VERY popular and effective. And yes!, the scope can see through the thumb…..idiots. Great insight about the rifle, .308 rifles can be a whole new learning curve for those entering this division. Good luck this season!

  • Rusty April 20, 2017, 11:39 am

    Can your scope see through your thumb? Impressive.
    I love the band wagon types who jump on the latest gun hotness. People need to remember that there is a difference between being a gun enthusiast and a ST member.

  • Pseudo April 4, 2017, 8:43 am

    Just a minor comment and yes I know many opt to install any manner of sights or scopes, but for these prices it would be nice if a set of at least cheap sights came with the gun.

  • Malcolm April 3, 2017, 7:31 pm

    Looks like a nice rifle. But it is in the restricted category in Canada, and no doubt over 2K.

  • David April 3, 2017, 11:37 am

    The scope sees through your hand?

  • Bob April 3, 2017, 9:46 am

    Just because you saw it in a video game … doesn’t mean you have to hold a rifle like a douche bag .

    • nunya April 3, 2017, 1:01 pm

      lol, it does come off as trying to be cool, but i think it was suppose to indicate that this is kind of designed for the 3gun comp heavy shooters and 3gunners tend to hold their rifles like that no?

      • Jon Hodoway April 4, 2017, 10:20 pm

        Not trying to be cool. I gave that up when I had kids.

    • Jon Hodoway April 4, 2017, 10:18 pm

      I checked and the “C-Clamp” grip is not how you hold a douche bag. It should be inserted using thumb and forefinger. I understand your confusion with this. I hope this helps. I am a little confused with what video game you are playing that uses a douche bag? Never mind I don’t want to know.
      Thanks for your comment


      • Big John April 13, 2017, 8:11 am

        +1 Jon…well done with the suttle well deserved bitch slap, and thank you for another great article.

  • Rick April 3, 2017, 9:15 am

    I wonder what kind of varmit was driving that SUV?

  • Will Drider March 29, 2017, 10:30 pm

    Deep Dive
    Its nice to see someone else using a torque wrench on scope mounts. The problem remains: how do you know the fasteners haven’t backed off in the field, after a flight and rough baggage handling or after 100 rounds with out the torque wrench?

    There is a product called touque seal or slipmark. It is a thick fluid in a tube that comes in several colors. You squeeze/piant a srtight line across the fastener and continue on the base. The material hardens. It is not a form of loc tite rather it provides a visual reference that the fastner has not moved. Movement would crack the marked line and would show misalignment of the split line the further it moves. This allows you to simply visually inspect the mounts without tools and know the applied torque has not backed off.

    This might not have value if your swapping scopes/rifles often but forlong term mounting it can help you detect a problem before “The Shot”.

    • BA April 3, 2017, 5:21 pm

      Little bit of clear nail polish will do the same thing, it will keep screws from backing themselves out from vibration. Just a dab on threads like loctite.

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