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

    Steadicam Gun Operator
    Night Vision & Thermal Aficionado
    Flashlight/Laser Enthusiast
    USPSA competitor

    Any questions please email him at [email protected]