Gear Review: SIG SAUER MCX Barrel Change Kit (300 AAC Blackout)

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.


  1. avatar Chadwick says:

    I can’t justify(or afford to justify) getting a SIG like that. I would probably get an AR pistol and settle for a few features not being as nice or convenient. Now that’s out of the way… That is a sweet looking setup you have there! You have me interested at least more than I was, and not in a caliber change kit.

  2. avatar Soylent Green says:

    “In this case that’s a new barrel, a new bolt (just in case your MCX was a .458 SOCOM version, I suppose)”

    I’m betting the new bolt is included because the headspace on the new barrel was verified correct with the supplied bolt. Even though it’s almost likely also good with your existing bolt, this way Sig knows and can prove it.

  3. avatar kevin says:

    There’s apparently a risk of a kaboom if you chamber a .300blk into a 5.56 barrel. Changing barrels on the fly would seem to enhance this issue. I’m picturing a cluttered table at a range with mixed calibers lying around. Anyone else see this as an issue?

    1. avatar jwtaylor says:

      Yes, but only because the 2nd amendment also applies to people too stupid or too inattentive to own a firearm.

      1. avatar Renner says:

        Automobiles, lawn mowers, politics, children, etc. Half the population will always have below average intelligence.

        1. avatar VerendusAudeo says:

          Just look at how stupid the average person is. Then remember that half of the population is dumber than that.

        2. avatar Yo Daddy says:

          Not if the population is 3.

      2. avatar Warren says:

        And honestly, if you’re not paying close enough attention to what you’re sticking in your gun, you kind of have it coming. That goes for any platform, any caliber. Shooting is not a hobby that plays kindly to negligence and inattention.

      3. avatar kevin says:

        I know smart and well trained people who have had ND’s and Kabooms. Its unwise to think that mistakes are just for “stupid” people.
        Jon, didn’t you just write about an ND?

        1. avatar jwtaylor says:

          Yup. I don’t remember writing about inserting the wrong caliber.

        2. avatar California Richard says:

          Its been my experience (between the Marine Corps and other ventures) that there are LOTS of well trained inattentive, lackadaisical, idiots…. just like colleges crank out educated idiots these days….. I always use the “Monster’s University” mantra when it comes training/education: “We’re here to make good monsters great, not mediocre monsters less mediocre.”

      4. avatar Yo Daddy says:

        And the 1st applies to people to know there’s a difference between rights and abilities.

    2. avatar Renner says:

      I don’t see it as being any more of an issue than having two dedicated uppers or two separate rifles for that matter. I use very clear markings on my blackout rifles and magazines to minimize the chances of a mistake. Since my son also uses these rifles, I’ve ingrained in him how important it is to pay attention to this as well as the usual safety rules.

    3. avatar Ian says:

      Faxon makes color coded and labeled bands for mags or a hand guard if that helps.

      Either that or we make a 5th rule of firearms safety: you’re responsible for your guns, ammo, and safety.

    4. avatar Yo Daddy says:

      I imagine it could cause a problem but then again I’m not sure the necked up cases and .308″ bullets are going to fit into the far end of the 5.56 chamber too well. I don’t have a .300 Copout but I do have a 7.62x40WT with is the same general idea with a case 5mm longer than .300. I also have a 5.56 and to be honest it never occurred to me to even try to see if they’ll fit. I honestly don’t THINK it’s going to go as smoothly as a person thinks, because I believe the barrel extensions are quite a big different with “slots” for the bullets that are considerably bigger for the .308s so I suspect a ,308 heading across a 5.56 extension is going to get ramped toward the roof of the chamber. Where it will probably get wedged and keep the bolt from going any further. I don’t think it will just slide up in their like it belongs there. But I could be wrong. I know my 40WT barrel extension is a lot different than my 5.56, but I don’t know about .300. Overall, I’d be really surprised if you can just slip a mag full of .300 BLK in and slap the bolt release and have a round in the chamber and a bolt in battery ready to fire.

  4. avatar Matt in Oklahoma says:

    That’s too high a cost for a shifting POA for me. I can build an AR for that. Good review though.

  5. avatar jwtaylor says:

    Seems like a well executed product, but at that price, why?
    For the same price, you can build or buy a decent upper, which will swap faster, maintain zero with whatever optic you already have on it, have the appropriate length hand guard for barrel length, and already have the silencer screwed on.
    Losing that seems like a very high price to pay for having a smaller storage footprint.

  6. avatar Shawn Graber says:

    Pricey caliber changeout kits…it appears SIG shares that foible with the IWI Tavor. Cool option to have; one weapon platform that can swap various calibers, (IWI is still working on their 300 blackout kit) but the gripes are on point: “I could build an AR-15 upper for cheaper than that!”

  7. avatar NorincoJay says:

    Get the price down to $350 with a bolt and spring. Have several calibers 6.5 Grendel, 6.8 SPC2, 300 blkout, 7.62×39 and a big bore caliber. I’m assuming the barrel is CHF. Maybe then it’d be worth having an MCX for the purpose of a quick switch. If you already have an MCX it’s your only way to swap calibers.

    As it is just a AR15 is still the way to go if you want to do caliber conversions. There is such a variety of AR15 calibers. All the part commonality and so easy to switch uppers around. Not to mention good prices.

  8. avatar Gunrunner says:

    Yes, you can “build an upper for less than that,” but unless you invest in another optic and mount to leave on the other upper (there goes the price argument), you’re still going to have some level of re-zeroing (probably more than 1 MOA) moving the optic between uppers.

    1. avatar California Richard says:

      QD mount, masking tape, sharpie pen. Write the poa shift for that upper on the tape and stick it on the inside of the scope cover. Military/police have been doing this for decades.

    2. avatar Yo Daddy says:

      He said the rail keeps the optic on the upper so the barrels can be switched. That’s why he has a POI shift he took the pictures of and was curious about.

  9. avatar JasonM says:

    Maybe one day they’ll have the MPX barrel change kits they’ve been promising us for two years.

  10. avatar VF 1777 says:

    At one point in time, this was my ‘grail gun’. A piston AAC side folder. I had my cash all ready to go and was gonna jump in blindfolded with both feet. But then came all of the hushed reports of ‘issues’ and all of the delays with no real clear information as to what the actual issues and fixes were… It was just too much for me to buy in – especially as an early adopter, and especially with a piston running 300 AAC, so I stopped my quest. It all got me thinking – there has to be a reason that everyone else has gone DI on their 300 shorties, right?. Heck, even LWRC made their first ever DI gun just for 300 AAC. So I’ll just sit back and watch. Who knows, maybe this will be the ‘gold standard’ some day and then maybe I’ll think about ponying up all that coin again. In the meantime, I’m quite happy with a (pre-relo) AAC MPW9 in 300 AAC. It shoots dimes, quite literally. I’ve seen them sell on GB for a crazy amount of cash. I wouldn’t sell mine for 2k right now. Let’s see if the MCX shortie in 300 can measure up to that in a few years of being out there in the wild… Glad to see they’re finally shipping 300 AAC shortie barrels now though. Ha, only took 2 years…

  11. avatar Have Blue says:

    Update for the author… Currently, SIG only ships a barrel. When I called to ask why, they said the new bolt and springs were no longer necessary.

    I really like the MCX side folding stock for an SBR. There are some clever short buffer tube stock options for gas guns. However, those seemed very expensive for a solution that doesn’t seem as robust as the MCX.

    I would not buy an MCX to be a 16″ 556 rifle. I do think there are better options for that. However, If you’re looking for a Suppressed 300BLK SBR, the MCX is the bee’s knees.

    1. avatar VF 1777 says:

      Yeah, on paper at least…

  12. avatar Grant says:

    This looks kinda neat, but doing caliber swaps isn’t really worth it if you have to re-zero your sights. Especially not for $600. I know AR’s are boring because everybody has one, but you can’t beat the prices. I just bought a Colt OEM1 for $675 shipped. It’s a Colt 6920 rifle minus the front hand guard, stock and trigger guard. I had an extra stock lying around (who doesn’t) and had to spring for a MOE hand guard for another $25. Seems like a better deal than a $600 barrel and gas piston.

    Sig has the same problem with their 320. Doing a caliber swap is a waste of money. $400 for a caliber change kit is pretty stupid when you can buy a complete handgun from Glock, FN, Beretta, etc for $500. Especially since they won’t sell you an extra chassis for $100. Sig needs to set their pricing so you can go to a stocking dealer and pick any frame, slide, barrel, chassis and 2 magazines for the same price as a new pistol.

    To me, the only swap that makes since on the 320 is to buy a 9mm compact or carry with the mid sized slide. Then you can switch between the 17 round carry frame, the 15 round compact and the 12 round subcompact for a semi reasonable price. I know the frames are supposed to sell for $50 and I’ve seen 3 packs of P320 mags for $96. So I’ve actually thought about buying the Compact and getting a Subcompact frame and a 12 round magazine for less than $100.

  13. avatar Geoff PR says:

    “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.”

    Eh, I’m only familiar with ‘doing a solid’ as meaning a solid favor, IE, a good thing.

    ” 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.”

    Or do I misunderstand the meaning of ‘a solid’?

  14. avatar Lefty Jason says:

    Why did you have to bring up Torchys Tacos? i live in salt lake city now and cant get torchys and now im craving torchys. i miss torchys.

  15. avatar Scoobydoo says:

    Maybe they should put those barrels on backordered SBRs….

    I ordered a FDE MCX SBR in 300BO last April and still haven’t seen it from SIG, let alone the ATF wait.

  16. avatar Yo Daddy says:

    Its certainly sexy but it also looks ilke about as much work or more work to swap than pulling a handguard, gas block, tube, nut and barrel. I imagine the screws are just clamping the gas system around the barrel, but how many are there and what all has to come apart?

    I’m also not a gunsmith but I’m a mechanic, lifelong gearhead and lover of anything that turns money into noise. And I’ve hammered together 10 or 12 ARs over the years including a pistol. So in my opinion, your shift in POI may be worth looking into. In particular I’d look for a torque spec or sequence for those clamp bolts and make sure everything goes in nice and square and stays that way during the tightening process. I don’t know how you went about it but I’d want the upper off the lower and the BCG and such out of the way so there is nothing the barrel is up against. You might also want to check the bore or threads or whatever in the hole the parts are sliding into and the parts themvselves for dirt, debris, carbon, burrs or anything that could prevent it from being square. You really shouldn’t have POI unless something is in there just a little bit crooked, and you might want to check that there’s nothing in there LIKE dirt or debris that could get blasted or shook out of where it is and create clearance that could let the barrel work around in there or let gas get somewhere it shouldn’t etc.

    Maybe convert it back and see if your POI shifts back. Maybe it was off before and you didn’t know it.

    1. avatar EMS/FD says:

      The article says it’s a 1 MOA shift. Is he talking about just taking the same barrel off and replacing it again or actually changing calibers from 5.56 to 300 blackout? The POI is going to be different between the 2 calibers and at different ranges. I didn’t see much difference between changing an upper, except changing an upper is cheaper and the parts are a whole lot more plentiful.

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