SIG MCX VIRTUS 300 AAC Swappable Barrel Review

Over the last 9 months of testing, the SIG MCX VIRTUS has proven to be a very reliable and useful weapon. In its 5.56 configurations, we saw it run in all weather conditions over the winter, a 2000 rounds suppressed test and many other shenanigans. The most impressive part to me was that we had a multi-configurable rifle, that consistently shot ½ MOA groups. This week, we finally got the long awaited 300 AAC barrel.

One of the great things about the MCX is the ease of barrel swaps. Two set screws hold the barrel in, torqued to 60-inch pounds. It takes about 2 minutes for a swap, which includes the caliber change to 300 AAC. 300 is a weird round, for certain. I know some people that use it for hunting, but I foresee a growing military niche as well. As the MCX was built from day one with the idea of being a military rifle, it makes sense they would want that capability.

For reasons I cannot explain, the 300 AAC never delivers accuracy similar to its 5.56 parent cartridge. My best guess is that the barrel twist rate is a compromise to stabilize both the super and subsonic variations of the projectile. The weight difference in this particular cartridge is staggering. Supersonic variants go as light as 95-grain solid copper, with a 125 grain being normal. Supersonic bullets are almost twice that, with 220 being pretty standard. A rifle manufacturer has used a barrel that will stabilize both, lest customer service become a war zone.

At any rate, the acceptable group size for 300 AAC tends to be a little bigger. This is helped by the fact that the round is near useless outside of 300 meters. Not a big deal, if you are using the round for its intended purpose. Which I still see as melting jihadi’s at room distances, or urban combat distances at worst. 3 MOA becomes less of a problem under night vision goggles and lasers.

So, we set out to check the accuracy on the 300 AAC barrel. Using SIG ammunition in super and subsonic configurations, 125 and 220 respectively, we went to work. Given past performance, I did the testing at 50 meters. Working against the rifle, I used only a 6x scope, and did the test on a windy day. The wind was causing the target to wiggle a bit, so we can call the test groups a bit on the larger side.

Supersonic ammunition produced groups of about 1 inch, which translates to 2 MOA. It liked the subsonic just a little bit less, turning in 1.5 inches, which translates to 3 MOA. For caliber, not a bad day out. And most important, the rifle ate everything we threw at it. The caliber swap gives the MCX unparalleled versatility, now we just need to petition SIG to make a barrel in 224 Valkyrie.

***Shop GunsAmerica for your next Sig MCX rifle***

About the author: Clay Martin is a former Marine and Green Beret, retiring out of 3rd Special Forces Group. He is a multi-decade and -service sniper, as well as 3-Gun competitor and Master ranked shooter in USPSA Production. In addition to writing about guns, he is the author of “Last Son of The War God,” a novel about shooting people that deserve it. You can also follow him on twitter, @offthe_res or his website,

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  • Bob Z December 19, 2020, 4:12 am

    Yeap… so much potential…
    1) 224 Valkyrie,
    2) Wilson Combat’s 300 H’AMR with 18″ stainless match grade bull barrel for DMR setups.
    3) Complete upper in 7.62 x 39 with tuned bolt and ejector (even Windham and PSA have these in DI)
    Mouth watering yet?
    4) Complete upper in .458 SOCOM (licensed by Tromix) with matching suppressor made by Sig.
    I guess we can dream.

  • Mike June 11, 2018, 6:31 pm

    My 84 yr old Mom has a 300 with 3 shot group touching. Burris extreme mounts, leupold vx2 2 x 7 – 33, w a German #1 reticle. Mount tightened to 55” lbs, 1/7 twist 150 gr Winchester whitetail extreme. Custom bolt n trigger job. Mom still deer n hog hunts every year.

  • I Love Liberty June 11, 2018, 4:17 pm

    I am in the process of getting a supersonic .300 AAC Blackout AR-15. I will only shoot 110 to 125 grain bullets out of mine. Before when the AR-15 was only in less effective 5.56 x 45 I had no desire to get one. I have yet to try the cartridge but from what I have read the extreme range of the .300 AAC Blackout rifle from a sixteen inch barrel with supersonic ammo is 280 to 300 yards as you stated in the article. I believe bullet drop is about 18 inches at 275 yards. The 5.56 round is not very effective at these ranges either as it has lost a lot of velocity.

  • C Keethler June 11, 2018, 9:27 am

    I would like to know what the twist rate is in this barrel, I have a 300 black-out in 16 in. I plan to run 190 gr. sub sonic in suppressed. My barrel has a 1 in 7 twist.

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