B5 Systems

SIG SAUER Launches MCX Rattler

I knew SIG had been working in a new, compact version of MCX. They’d been refining this design for awhile, having heard rumor of it overseas as far back as two years ago during DSEI. Later, after asking about it, I was shown developmental models. Today, it all becomes clear. SIG SAUER is launching the Rattler, the latest addition to the MCX family. I got a chance to speak briefly about the gun with Patrick Hanley, Rifle Product Manager at SIG.

The first question I asked him was about the name. I’d been hearing it float around for quite awhile and wondered where it came from. Apparently, after firing it on full-auto the first time, the designer said, “It really rattles your teeth when you fire it.” Considering the nickname of MCX, Rattler stuck.

Next, I asked him why they built it. Hanley answered, “Everything was designed to be as discrete and small as possible with this gun.” Quite simply, customers came to them and asked for a Subgun size weapon in 5.56mm or .300 BLK. This is the result of that work.

Initially, they leveraged the MCX short stroke piston design, but quickly found that the upper would have been longer than the 5.5″ barrel. Instead, they developed two separate gas systems, one for the .300 and a second for the 5.56.

The .300 gas system has two positions and can be used suppressed. The 5.56 is a plugged gas system to prevent valve corrosion. Unfortunately, it cannot be used with a suppressor; not at that length. However, swapping between calibers, is as simple as swapping barrels.

The stock was designed so that all controls can be manipulated with it closed and the weapon itself can be fired with the stock closed, making it a very compact package. Additionally, the Rattler’s upper receiver has no forward assist and their slick side deflector for spent cases.

SIG may have succeeded in creating the smallest production AR-style weapon in the world. It’s 16″ while folded and 23.5″ with the stock extended.

Based on these differences, the Rattler comes as a complete pistol (with SPB) or SBR. The barrels are not compatible with any other MCX upper, so SIG is offering dedicated Rattler Upper Receiver Kits which come with the AR lower adapter knuckle. The Rattler upper kit can be used with an MCX lower or AR lower, when used with the adapter knuckle.  For owners of Rattlers, barrel kits to swap calibers will be available so they don’t have to purchase complete URGs. 

The MCX Rattler launches today.

UPDATE: .300 Rattlers will ship next week and 5.56 in November. 



26 Responses to “SIG SAUER Launches MCX Rattler”

  1. charlie taylor says:

    “Additionally, the Rattler’s upper receiver has forward assist and their slick side deflector for spent cases.”

    I believe you meant to say has no forward assist, not that it has one.

  2. Bad Dancer says:

    Dawgonnit take my money now.

  3. Andres says:

    Can anyone specify as to the sbr difference? Does the barrel length remain the saim with the stock picktured?

    • Andres says:

      Upon taking a second look at the pictures and gawking at my spelling, it appears there is in fact two different stocks in the pictures. A folding stock, and pdw style collapsing stock.

  4. James says:

    Does this fill all the boxes for the NSW pdw ?

  5. Kemp says:

    What kind of ballistics are we looking at from a 5″ barrel? Do you lose much compared to 7.5″?

    • MP says:

      The existing 300blk barrel is 6.75″.
      The 5″ barrel has an extra twist in it and by all accounts very accurate for such a small gun.

      • DG says:

        Where did you see the twist rate posted? I own the 6.75″ 300blk and honestly won’t shoot supers because of folks reporting baffle strikes with the lighter supers due to the 1:5 twist

  6. SVGC says:

    Will SIG be releasing the adapter knuckles or any other MCX uppers to the general public in the near future?

    • DG says:

      Near future? Wouldn’t hold my breath considering they’ve teased that knuckle for almost two years now.

      • SSD says:

        I’ve been on them about it. They are supposed to come with the URGs.

        • DG says:

          My big question now is with all these changes made during the MCX recall and all the issues folks are having with different trigger groups, would the adapter on an AR lower even still function with the newer BCG and firing pin safety on the MCX?

  7. TominVA says:

    What would the max effective range be on this? 200m?

    • mark says:

      If we’re defining effective range by expansion/frag range then:
      Barnes 110g Tac-TX has an 1400fps expansion threshold, w/ 0.289 G1 BC.

      6.75″ barrel .300blk = 1850fps. Lets say 5.5″ is 1750fps.

      –> 1419fps/ 491 ftlbs @ 150 yards.

  8. Bordertop says:


    $2,800 bucks.

    Lemme guess, has SIG proprietary parts in the lower too. So you can NEVER get parts for your overpriced toy.

    • Jeff S says:

      Be sure to stock up on the parts kits as SIG won’t make them forever. 🙁


      The owner of two SIG556 models

  9. Swiss Pete says:

    Sooo, “discrete” but not usable with a suppressor in .223? How does that compute?

    Still want one, but in .300 Blackout


    • James says:

      Twist rate is longer than the barrel, barely stabilizes the bullet increasing the chance of baffle strikes(not to mention increased wear on the blast baffle with short barrels). Almost all suppressor mfg recommend against 5.56 barrels less than 10″ being used with their cans. Some take it seriously enough to void warranties.

  10. Second Grade Math says:

    Why does it cost almost twice as much as a regular MCX?