Breaking News: Sig MCX Rattler AKA MCX-K

Earlier this year US SOCOM submitted an RFI for a new PDW. Nathaniel F. wrote an article about it back in March.

PDW Kit specifics: The kit must be adaptable to the standard M4A1 lower receiver, any modification to the lower receiver must be reversible and nonpermanent. The kit must be in .300 Blackout (BLK) cartridge, total system weight, including the M4A1 lower is not to exceed 5.5lbs. Length with stock extended not to extend 26″ length with stock collapsed or folded shall be 17″ (T), 15″ (O) and a height not to exceed 7.5″. Weapon shall be fully functional when collapsed or folded. Kit should include a 5.56mm barrel that can be changed from .300 BLK to 5.56mm in less than 3 minutes. Accuracy shall be 3.0 MOA (T), 2.0 MOA (O) @100 yds. and 5.0 MOA (T), 3.0 MOA (O) @ 300 yds. both in .300 BLK supersonic.

Well Sig Sauer is coming out with something that looks like it fits the bill. The Sig MCX Rattler. I call it the MCX-K. Like an MP5 vs MP5K, the MCX Rattler is a tiny version of a Sig MCX. However it is not just a shorter barrel version of an MCX or MCX Virtus.

Looking at the right hand side of the MCX Virtus it has a forward assist and a similar brass deflector like on the regular MCX.

The Sig MCX Virtus pistol has a barrel length of 9″ whereas the Rattler is only 5.5″ making the OAL only 16″ when folded.


My MPX pistol next to my friend’s MCX pistol in .300blk. (Yes the muzzle brake is not timed because we were just testing a suppressor and it was removed for the photo)



Below is a comparison of an early Rattler prototype.

Top – Bottom: MCX SBR, MP5SD, MP5K, MCX Rattler


The earlier prototypes have Keymod handguards but the new Rattler looks to have MLOK. Notice the lack of a forward assist. Even the brass deflector has been neutered a bit.


This is a five shot group of .300blk super sonic at 300 yards from the 5.5″ MCX Rattler. I didn’t see the one on the black line earlier. This was shot and documented at Sig Academy.


I hope they release the Rattler as a pistol. I am not sure why anyone would want the Virtus pistol over this thing though.

Edit: In the RFI above, it states that US SOCOM is looking for a conversion kit to convert existing M4A1 lowers. Sig has an AR lower receiver adapter that allows you to use an MCX stock and MCX upper receiver on your AR-15.

You can see what looks like a 516-MCX hybrid.


Edit: Sig Sauer has the Rattler up on their website. It will be available as a pistol and an SBR with an MSRP of $2,700. At the moment, it is only being offered in .300blk. Not sure if the 5.56 version will be available later. when we find out we will update you all.

Here is a video showcasing the Rattler.

Nicholas C

Co-Founder of KRISSTALK forums, an owner’s support group and all things KRISS Vector related. Nick found his passion through competitive shooting while living in NY. He participates in USPSA and 3Gun. He loves all things that shoots and flashlights. Really really bright flashlights.

Any questions please email him at


  • CoastieGM

    Cue the “yea but is it drop safe” snark machine.

    • Thomas Bennett

      Yeah, but is that Snark machine, drop safe?

      • mcjagermech

        Hit it with a Brownells hammer, it’s the only way to be sure

        • PK

          …is the Brownells hammer drop safe?

          • mcjagermech

            Only one way to find out

  • ForTehNguyen

    what barrel length was that grouping on

    • Nicholas C


    • Rocky Mountain 9

      The Rattler barrel, apparently. You can see a fifth bullet nearly hidden in the black line, making it a roughly 3″, five shot group at 300 yards.

    • Crazy but 5.5 inch barrel

      • PK

        Why crazy? Short barrels are stiffer, given identical materials and the same diameter. I’ve often seen groups tighten substantially by going from 24″ barrels to 18″ barrels, and the effect hardly stops at any particular length.

  • Ulysses

    “I am not sure why anyone would want the Virtus pistol over this thing though.”
    Nobody should want either in 5.56, that’s for sure.

  • AndyHasky

    I was hoping the announcement tomorrow would be for a .308 mcx, instead we’re getting this I guess.

    • KidCorporate

      Same here. That would be soooo hot.

  • Arnold Llw

    What’s the twist rate? I’ve read reports that the MCX SBR’s slower 1:5 is causing havoc with lighter supersonic rounds

    • Stewart Hickey

      Did you mean faster? 1:5 is a ridiculously fast twist rate.

      • Arnold Llw

        Oops you’re correct! Meant faster!

        • FarmerB

          That’s for 300 only though, right? The 5.56 are still 1:7, I think.

          • Arnold Llw

            300 MCX Carbine is 1:6
            300 MCX SBR is 1:5.
            5.56 MCX is 1:7

  • What’s the velocity for a 110gr .300 BLK from a 5.5″ barrel?

    • Cyborg Fred

      About 1800fps

  • feetpiece _

    It would be super if FN just made a SCAR 16 compatible upper.

    Problem solved and Sig gets to find another way to pay for P320 settlements.

    • JumpIf NotZero

      What are you talking about? FN’s SCAR upper is the firearm.

      • noob

        make an adapter that joins a M4 lower to a SCAR upper, so you can firearm while you firearm.

  • SD

    M-lok on the MCX? Beautiful!!!

    • Nicholas C

      You could get a Midwest Industries MLOK handguard for your MCX.

      • xxxxxxxxxxesntetismskskskskrks

        Lancer Systems makes a carbon fiber handguard aswell

        • Nicholas C

          Yes but they dont have a top rail.

      • Aren’t those friction-mount, though?

        • Nicholas C

          Not sure what you mean. The MI handguard and the Lancer lock onto the front takedown pin.

  • The_Champ

    From the advertisement “M4 ballistics in a subgun-sized package”


    • Jack

      Wondering what kind of velocity/energy your getting from 5.56 in a 5.5″ barrel. I also thought the 6.75″ MCX barrel in 300 was a little short. Guess I was wrong.

      • CommonSense23

        The only round I can think of is the Black hills 70gr. That load is made for 10 inches and expands at pretty low velocities.

      • Goosey

        5.56 and 5.5″, mid 500 ft-lb range.

        Seems kinda dumb to me. But neat!

        • ostiariusalpha

          So, a little more energy than a 124gr +P+ 9x19mm from a 4.5″ barrel, but only 76% of the momentum. At least you’ll be able to break the skin, I guess.

        • Paelorian

          With a 5.5″ barrel, it’s useless in 5.56mm. We’re talking superior performance from pistol rounds. At least in 300BLK it still has energy about on par with the typical 5.56 10″ SBR. If designing from the ground up and without logistical and training concerns a powerful pistol round like 10mm would be more sensible. If length with the stock folded is not the critical dimension (as it is concealed carry, like in shoulder holster or briefcase), and the concern is compactness when in use, then a bullpup SBR like the the IWI X95 appears to be shorter than the Sig Rattler with the stock deployed (I don’t have the overall length with deployed stock from Sig), yet it actually delivers M4 ballistics. Most X95s have a 13″ barrel with 22.8″ overall length (OAL) or 15″ barrel with 25.2″ OAL. For comparison, the FN P90 has an OAL of 20″ and is used by the US Secret Service. There are 300BLK variants of the X95 that make that 13″ barrel have energy from the muzzle slightly exceeding the energy from a 20″ 5.56mm barrel. The man rappelling in the photograph is obviously not incognito and has no need to hide his weapon, so he would doubtless be better armed and more capable with a bullpup. The 13″ barrel X95 is less than 3″ longer than the P90 concealed by the US Secret Service. Cut it down 3″, adding a vertical grip to compensate for the short handguard, and you’d have a 10″ barrel 5.56 or 300BLK shorter than a PS90 and with very respectable ballistics that is a far more capable weapon than the 16″ folded stock OAL Rattler. The only niche I can see this practical in is for who are carrying a firearm concealed under a jacket, desire more firepower than a pistol, and don’t mind carrying a stock that needs to be deployed before use, and for whom discretion is as important as protection. For those people 4″ of added length could easily be a dealbreaker, and so a folding stock SMG or SBR can still make sense. The MCX isn’t even that much lighter than a bullpup of similar OAL. Maybe a pound more. The 9″ barrel MCX SBR is 5.8lbs. A theoretical X95 with a 9″ barrel would be roughly a pound more, as the 13″ barrel variant is 7.3lbs. Yet the X95 is not a particularly light bullpup.

      • mig1nc

        .300Blk typically has about 60% greater energy than 5.56mm, so think of 5.5″ .300Blk as being like a nearly 9″ 5.56mm. Like a G36C or SG552.

        • noob

          Makes sense: bigger diameter, means bigger swept volume and better performance in short barrels. Kinda like how the russians went to 9mm suppressed rifles.

      • Oh, three or four.

  • 360_AD

    Compatible with AR lower? Hm, how much does a MCX upper go for and do they even offer just uppers?

    • Anonymoose

      You need the buffer tube plug or it can chew up your lower. They haven’t released the plugs for sale yet afaik.

      • KidCorporate

        What are the plugs made of? Couldn’t be too hard to machine something up…

        • Anonymoose

          Aluminum I think.

  • Cyborg Fred
  • Haulin’ Oats

    I’m still waiting for a 10mm and 357sig version for home defense.

    • Nicholas C

      Why not a 10mm KRISS Vector?

      • PK

        I’m still so salty that there aren’t any factory extended mags. Everyone makes 9x19mm and 40S&W mags in tons of lengths/capacities, and 10x25mm is stuck with pistol capacity.

        For me, that’s the reason I don’t have a Vector in 10mm, yet.

      • Haulin’ Oats

        Vectors are great but where can I find a 30 round 10mm glock magazine? Also some competition in this space would be nice.

        • Nicholas C

          You have to do a little DIY and modify the .45acp mag extensions and magazine to make them work.

          • Kivaari

            Are they as hard to load as the 45 magazines. A 20+ magazine that you can get around 10 rounds into isn’t a big advantage.

  • RSG

    I’m skeptical of the 3″ group at 300 yards. But besides that, as a big believer and very early adopter of 300blk, I say rock on. I want one.

    • JumpIf NotZero

      Why? Barrel length has nothing to do with accuracy. What you should be asking is what the elevation adjustment was.

      • RSG

        Not speaking about barrel length. I’m speaking about Sig’s quality of rifles in general and the MCX specifically. I’ve not seen one review that has shown the MCX to be an MOA rifle. Even harder still on a swappable barrel platform. I didn’t say was necessarily lying, just that I’m skeptical. That means I’ll need further verification.

  • Vitor Roma

    5.56 out of 5.5″ bbl is so dumb it’s funny. Hell, evem .300blk seems to lose a lot from such short bbl.

  • Class03180S

    “I am not sure why anyone would want the Virtus pistol over this thing though.”

    Maybe because you can’t suppress it? According to SIG: ” The 5.56 is a plugged gas system to prevent valve corrosion. Unfortunately, it cannot be used with a suppressor.” So you get a 5.56 with neutered ballistics AND you can’t even suppress it.
    But it is small. So there’s that.

    • kalashnikev

      Even if SIG said you could, I don’t think any 5.56 cans are rated to go quite that stupid short.

      • FarmerB

        5.56 cans on 5.5″ long barrels are called – barrels!!

        Actually – can we confirm 5.56 will be available – I’m hearing 300 only?

      • PK

        Some, I’m sure, can be made that will withstand that. I have shot a 30-06 pistol with a can, and the barrel is only about 8″. It’s not exactly a light silencer, though… nor is it semi/full auto.

        The muzzle pressure on a 5.5″ barrel should be about… 20750psi. Ouch. Not quite double the pressure from a 10.5″ barrel (11,500psi) but close.

    • Nicholas C

      I wouldn’t want the Rattler in 5.56 anyway. If I get one, it will be in .300blk. mmmm delicious.

  • Cal.Bar

    Yeah, we civilians were promised an adapter to put the sig MCX uppers on our M4 lowers years ago as well. Maybe after they sell a few million to the military, a few crumbs will fall our way.

  • iksnilol

    My teeth rattle from thinking of firing that thing.

    • PK

      I know! That’d be quite the muzzle blast.

      I want one too.

    • Nicholas C

      Not if you use a blast jacket like a Surefire Warden or a Noveske Pig.

      • iksnilol

        Still, there’s gonna be hella noise. You’re using a pistol barrel on a rifle cartridge.

      • kalashnikev

        Or… instead of hanging long, heavy steel off your muzzle, you could just source a barrel that’s not goofball-length to begin with.

        • Nicholas C

          I’m guessing you haven’t shot with blast jacket before. They are not that heavy. Lighter than hanging a suppressor.

          Also you may want a longer barrel but this weapon has a very specific purpose due to its size.

          • kalashnikev

            I’m familiar with both devices, and the KX3 is, IMO quite heavy. Purpose of a “Blast Jacket” is to give dudes in ban-states something to fiddle-F with up by the muzzle, like they own a suppressor… but they don’t. Purpose of the KX3 is the same as a Krink booster- to increase pressure to correct for improperly spec’d barrel.

  • Ray Scarbrough

    Just ! What I’ve been waiting for !!~ .. More knockdown !.. Longer Range !.. Lighter !
    More Adaptable ! and Affordable !.. than an ( Mp5 )!!~ ….Can’t Wait !!~
    I Can’t See a Downside ! … Can You???
    SEND MINE ! … TODAY !!~

    • JumpIf NotZero

      It’s not cheaper to shoot, and it’s not as quiet as 9mm, ammo isn’t as available.

      300blk has benefits but it also has issues.

      • Blake

        I just can’t spend money on a large pistol. If I am going bigger than a concealable handgun, I want rifle ballistics and knockdown power. I don’t really see any point to PCC’s because I really think “train with what you use” is extremely important. I am also pretty excited by this. I might finally buy a small rifle like this.

        • PK

          Exactly, with supersonic ammo it would be quite the thing. AR ergonomics, 110gr at 1700FPS or so, approximately the size of an MP5K… and the option to act like a .45acp with subsonic ammo.

  • Nice.

    • ostiariusalpha

      …the same as .22 Mag from a 26″ barrel.

  • PK

    Why would they use a 1-in-5″ .308 bore barrel for 300BLK? That’s nuts.

    • Kivaari

      For real heavy bullets at low velocity. Keep the RPM high so they are stable. At 1:10 and slow speeds they will go unstable instantly.

      • PK

        Yes, but 1-in-6″ would do fine with 220gr at very slow speeds… without exploding the supersonic lightweight rounds.

        • Kivaari

          Realistically, the only reason for the 300 blk is for suppressed use. Building it for that serious use makes sense.

          • PK

            …except the whole “must be able to run supersonic” requirement. I get what you’re saying, but having the ability to switch between 110gr at 1600FPS and 220gr at 900FPS is like having both a fairly modest rifle and a .45acp handgun. If the same barrel doesn’t work for both broad types of ammo, it’s a problem.

          • Kivaari

            I just see no real purpose for the 300 Blk except for suppressed use. It is such a weenie when fired from such small guns. Then there is no reason to have one with a 16 inch barrel and no suppressor either. I think the whole switching ammo concept is just a waste of time. I’d rather have a 16″ 5.56mm with a can. Yes, I understand the concept of no sonic crack with the heavy 300 Blk. If that is what you need it does a good job. I just can’t get too wrapped up in it, even in a military setting the use is so minimal.

          • PK

            The SOCOM RFI specifies subsonic/supersonic.

  • hikerguy

    Since it appears the US Army is determined to go with a 7.62 main battle rifle, this would be a perfect fit for MPs, specops, vehicle riders and drivers, tankers,and for those who would be with the main riflemen for Close battle combat such as entry into buildings and other tight urban situations. Easier to maneuver with indoors that an 18 inch barreled AR variant. PDWs years ago were referred to as “An answer for the question never asked.” Maybe the question as finally arrived. Sure is an expensive solution. I am sure other vendors will soon put out their versions as well.
    Most likely the M4 will become what it was meant to be in the beginning……a PDW. This will be limited to specops.

  • iksnilol

    I feel this is like a worse version of the AS VAL or Vikhr (conceptually).

    300 BLK is simply too small for such a short barrel.

  • Joshua Graham

    Has the lower receiver adapter been released yet?

  • smcallahan8

    What does the (O) and the (T) stand for in the specs?

    • Rob

      Objective and Threashold. Objective is what they would like. Threashold is the minimum to be considered.

  • Mr.SATism

    Great for PMC’s, is it even necessary for civilians?

  • Stephen Summer

    500ft/lbs, via select fire. Its going to effective in CQB. And SOCOM does t use standard 556.

  • Lee Attiny

    Any reason you couldn’t modify a milspec carrier with sig’s receiver adapter and run any upper you want instead of buying sig’s upper?

  • DropGun25

    That was some pretty blunt racial stereotyping…Where’s the video showcasing Antifa trying to take back the streets? There’s your client base!