The AR-15 platform was “good enough” when it came to swapping calibers. Changing the actual barrels was a pain in the butt, but given how cheap upper receivers are to buy you could simply have a complete upper for every different caliber you wanted to shoot. That’s how I lived my life as an AR-15 owner with diverse tastes in calibers. But SIG SAUER apparently wasn’t happy with “good enough”.
The SIG SAUER MCX was designed so that the barrel itself could easily be swapped out for a new caliber (saving the cost of an upper receiver and space in your gun safe), and one of the first available is 300 AAC Blackout.
One of the major selling points of the MCX platform is the ability to quickly swap barrels. On the AR-15 the barrel is held in place with a barrel nut, and a gas tube is pinned in place keeping it from rotating too far. Removing the barrel means removing the handguard, gas tube, and barrel nut before even having the option to slot out the barrel itself. A major PITA.
With the MCX, the handguard is held in place with the forward takedown pin and slides freely once pulled. From there all you need to do is un-screw two captive screws (they don’t fall out when fully un-screwed) and retract the bolt to release the barrel. Slot in a new barrel of your choice, tighten things back down, and you’re good to go.
I get the feeling that SIG SAUER may have done me a solid and shipped my barrel exchange kit before any of the branding was ready. I get that it’s just a barrel kit, but given that people will be dropping somewhere around $600 a pop for these things a printed sleeve would be nice. Yes, that’s severe nit-picking, but there’s not much else to complain about here
The kit comes with exactly what you’d need to change your MCX from whatever caliber you’re currently running to the new one. In this case that’s a new barrel, a new bolt (just in case your MCX was a .458 SOCOM version, I suppose) and a new set of springs for your bolt carrier group. Also included: the ONE WRENCH TO RULE THEM ALL. That’s really the only tool you need to make the change, other than your own two hands.
Notably absent from the kit: a handguard. SIG SAUER sells the handguards seperately, allowing you to choose the one you prefer. There are a variety of lengths and styles available, but silencer owners beware: if you’re looking for that cool “integrated” look, the “standard” hand guard won’t accommodate a standard silencer under the rails. There’s a silencer-ready version of the handguard you’ll need instead that has a larger internal diameter.
Once configured you should be good to go. One of the big advantages to this system is that optics don’t have to change between calibers — the top rail is long enough to accommodate nearly any optic configuration, and since the upper receiver remains the same, there’s no driving need to swap.
In terms of point of impact shift, there definitely is some. I’m calling it about 1/2 inch at 50 yards or a 1 MoA shift. If time is so short that you don’t have time to re-zero your rifle between caliber swaps, you should be good enough as long as you can remember to adjust for the new trajectory of your projectiles. But when time a new re-zero is always a good idea.
Accuracy is pretty darn good. I don’t notice that much of a difference between this barrel and my Noveske 300 BLK setup, and if you ignore that flier off to the left (hey — it was a gusty day!) then what you see is a roughly sub MoA group.
Overall it’s a solid kit. The barrel is well made, the kit comes with all the parts you need to do the swap, and the process is easier than getting fat while living next to a Torchy’s Tacos.
Specifications: SIG SAUER MCX Conversion Kit
Caliber: .300 AAC Blackout (tested), 5.56 NATO
Price: $615 (on SIG SAUER’s website)
Ratings (out of five stars):
Overall: * * * *
Works as advertised. It’s still slightly more expensive and complicated than buying a completely new upper receiver for your AR-15 rifle, but I’m hoping that once the MCX becomes a more widely adopted firearm the economy of scale will start to bring that price down. Hopefully we’ll also see some aftermarket barrel options too.