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.

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