SIG/M4 Rattler – Convert Your Existing SBR To A Rattler

Perhaps the most popular new cartridge in years has been the 300 AAC BLACKOUT.  Shortly after it was made public, I had an opportunity to meet with Kevin Brittingham.  At the time, he was still with Advanced Armament Corporation.  He gave me the inside story of how AAC came to develop the new cartridge.  The 300 AAC BLACKOUT cartridge was the result of a Request for Proposal from a special mission unit within the United States military.  The unit wanted a .30 caliber cartridge that would utilize the standard 5.56 magazines and bolt.  The RFP also proposed both a sub-sonic round for use with a suppressor and a supersonic round.  Robert Silvers, Director of Research and Development for AAC, put pen to paper, or stylus to computer with the following objectives:

  • Create a reliable and compact .30 caliber round for the AR platform;
  • The round would utilize existing magazines and retain their full capacity;
  • Create a sub-sonic round that is optimized for sound and flash suppression;
  • Create a supersonic round that matches 7.63X39MM ballistics;
  • Provide the ability to penetrate barriers with a high-mass projectile, and
  • Provide a lightweight, durable, and low recoiling upper receiver.

The end result is what is now known as the 300 AAC BLACKOUT.  The 300 AAC BLACKOUT, or technically 7.62X35mm, has the same rim dimensions and cartridge taper as the 5.56.   This was a critical requirement in order to utilize the existing mil-spec 5.56 bolt and feed reliably from a standard 5.56 magazine.  The configuration allows a standard M-4 to be converted to 300 AAC BLACKOUT by simply changing the barrel.

A second requirement was to provide the operator with one rifle that could meet multiple mission requirements.  Sub-sonic ammunition would allow the 300 AAC BLK to be a quiet and effective CQB platform while delivering a .30 caliber, 220 grain projectile on target.  By changing to a supersonic round, the operator could have a fully capable battle rifle, effective to 300 yards and beyond.

Most of the carbine offerings were in two configurations, either an 8” short barrel rifle or a non-NFA 16” configuration.  The 9” barrel is considered an ideal length for a traditional direct impingement carbine. I purchased one of the AAC MPW SBR carbines, along with the 762-SDN-6 suppressor.  Both have proven reliable and the 762-SDN-6 has performed well with both supersonic and subsonic ammunition.  However, the MPW, like most BLACKOUT carbines, is based on the original M-16 design.  That design requires a recoil spring housed in a buffer tube extension.  This design limits how compact these carbines can be made.

In 2015, SIG introduced the SIG MCX series of carbines and changed the M-16/M-4 world.  The MCX utilizes a short stroke piston system with a captured recoil system that is contained within the upper receiver.  This allows the MCX family of carbines to be equipped with a number of stock configurations, to include a folding stock.  The MCX system has proven very successful and has been adopted by agencies.  One of these is the newly constituted unit within the London’s Metropolitan Police Counter Terrorist Specialist Firearms Officer program.  As cool as the MCX is, many considered it “just another black rifle.”

Then came the SIG Rattler, the smallest production carbine on the market today.  The Rattler uses the MCX operating system along with the same upper and lower receiver.  However, the Rattler features a 5.5” barrel and is chambered in 300 BLK.  The Rattler became the darling for those in need of a Personal Defense Weapon (PDW) for protective details and covert operations.  The Rattler also features an adjustable gas system that allows it to be run with and without a suppressor.

SIG’s SRD762Ti suppressor is the ideal suppressor for the PDW mission.  The SRD762Ti is made from Grade 5 Titanium baffles that are precisely welded without the use of an external tube.  This allows the 1.75” diameter suppressor to have more volume than traditional designs.  It is 9.25” in length and weighs in at just over 18 oz.  While ideal for the Rattler, it should be noted that the SRD762Ti is rated for .30 caliber cartridges up to 300 Winchester Magnum.  SIG offers the SRD752Ti in both a direct thread and quick detach configuration.  The SRD762Ti is a must-have accessory in order to make full operational use of the Rattler.

Over the last several years, there have been whispers that the special operations community, and specific units, have been utilizing carbines chambered in 300 BLK.  Reportedly, Black Hills manufactured a 110 TTSX load and a 201 gr. solid copper subsonic load for some of these units.  Those who watch the defense industry blogs may have seen several solicitation notices for 300 BLK ammo.  Recently, the FBI released a solicitation for ammunition for HRT.  It appears that the 300 BLK is no longer just a niche or special interest cartridge.

One of the more interesting solicitations from SOCOM was released in March of this year.  The pertinent information is copied below:

Personal Defense Weapon System Sources Sought

The United States Special Operations Command (USSOCOM) is seeking to identify potential sources within the national technology and industrial base with the ability to provide a conversion kit for the M4A1 to create a Personal Defense Weapon (PDW) system:

-Complete Kit to include upper receiver and buttstock kit
-Any tools required to complete an operational conversion
-A light and sound reducing suppressor that can be attached to the system when needed

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 in not to exceed 5.5lbs. Length with stock extended not to exceed 26″ length with stock collapsed or folded shall be 17″ (T), 15″ (O) and a height not to exceed 7.5″. The weapon shall be fully functional when collapsed or folded. The 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.

This announcement constitutes a Sources Sought. It is not a solicitation or request for proposal, but an effort to obtain information for planning purposes only.

The ability to convert existing M4A1 carbines to a PDW, chambered in 300 BLK, offers several appealing advantages.  First, it is far more economical than purchasing complete carbines.  Far more conversion kits can be purchased than complete carbines.  Second, the purchase of a retrofit kit presents fewer issues than building a new carbine.  Finally, should the receiver be damaged, the conversion kit can be fitted to another lower.

While I had been following the development of the Rattler for some time, I was not willing to go through the entire NFA Form 4 process.  This all changed when SIG introduced the Rattler as a pistol.  The pistol configuration featured a collapsible arm brace.  I ordered one of the pistols with the intention of using it in place of one of my 300 BLK SBRs.  Then came the SOCOM solicitation.  I contacted SIG and they were able to ship me the conversion kit.  I was now in business with a Rattler SBR.

The conversion kit consists of an adaptor block that replaces the standard buffer tube.  It is held in place with a large screw and extends approximately 1 ¼” from the end of the lower receiver.  The rear of the adaptor block is configured with a Picatinny style rail with three slots. 

The other component of the Rattler kit is the side folding stock.  The stock has a cross bolt mount that interfaces with the adaptor plate and is secured with a large hex bolt.  The folding stock extends approximately 7.5” from the rear of the adaptor and 9.25” from the rear of the upper receiver.  The Rattler upper receiver is a direct replacement for the standard M4 upper receiver.  It took me less than 10 minutes to convert one of my registered Smith & Wesson M&P SBR lowers to a full-blown Rattler!

I was immediately impressed with just how compact the Rattler is.  With the stock folded, the overall length is right at 16.25”!  With the stock extended, the Rattler is 23.75” in length. That is still a very short package.  The operating system of the Rattler incorporates an easily adjusted gas block for running supersonic and subsonic loads.

For an optic, I reached out to Aimpoint for one of the new CompM5 red dot optics.  The CompM5 is the latest generation of Micro size optics but is not in the Micro family.  The CompM5 was developed to be a ruggedized optic that can withstand the harshest operating environment.  The CompM5 is powered by a single AAA battery and eliminates the issues involved with flying with the CR123 lithium batteries.  It is fully compatible with all generations of night vision optics and Aimpoint’s 3X magnifiers.  For more details about the upgraded performance and electronics, visit Aimpoint’s website at 

I also installed an extra set of Troy Industry backup iron sights as a secondary means of shooting.  The sharp reader will notice several range photos where I have the front lens cover on the CompM5 closed.  This was intentional!  Shooting with both eyes open, the dot will superimpose itself on the target as with the Armson OEG sights.  Armson USA OEG

On the range, I tested the Rattler, from a bench rest, at 50-yards, using the CompM5 and the 3X magnifier.  A phone call to my partners at Federal, Hornady, and SIG allowed me to test the Rattler with seven different loads.  All bench testing was done with the SRD762Ti in place for both sound reduction and to test reliability.  The suppressed Rattler was 100% reliable with all of the loads and, as reflected in the accompanying chart, all loads produced groups less than 1”.  For full disclosure, I suffer from astigmatism and even the sharp dot on the CompM5 became pixilated.  This made maintaining a consistent point of aim difficult.  To offset shooter error, I measured the best three shots out of a five shot group.  I found the stock to be incredibly solid and it locked up with no play or shift.

I zeroed the Rattler for the new Hornady 190 Sub-X ammo, knowing that would be my “go to” round for dispatching critters etc.  During the range test, the first anomaly I noticed was that supersonic loads had not only an elevation shift from point of zero, but also a very noticeable windage shift.  In some cases, the windage shift was as great as 3 to 4 inches from point of aim.  The second item I found was that the Rattler would not function subsonic loads without a suppressor.  The gun short stroked every subsonic load, regardless of gas setting.  I was very impressed with the performance of the SRD762Ti suppressor when using both the supersonic and the amazingly quiet subsonic loads.

I contacted John Hollister, SIG guru and an all around good guy, to see what was going on.  John and I have been friends from his days at AAC and he has always been a straight shooter and never shied away from the tough questions.  John said that both issues were “normal” for the Rattler.  SIG worked very hard to gas the gun properly to run suppressed and unsuppressed, to ensure maximum reliability.  A byproduct is that the Rattler will not cycle most subsonic loads without the additional back pressure that is generated by a suppressor.  With regard to the shift in impact, John stated that the shift was the result of the different kinetic energy between the different rounds.  The supersonic rounds “whip” the barrel differently than the slower subsonic rounds and is not specific to just the Rattler.  This shift is predictable and one of the results of designing a small platform to run with bullet weights that go from 85 grains to 200 grains or heavier.  Given the true operating parameters of a PDW in a covert or PSD role, it is unlikely that the operator would be changing between to dramatically different loads.  Still, it is a drawback that should be understood and accounted for.

If I were setting up the Rattler for a protective detail, or a low profile mission, I would probably zero it with Federal’s 110 grain Solid Copper HP or the Hornady Black 110 V-Max load.  In a defensive environment, where keeping the weapon concealed is essential, there would be very little opportunity to install the suppressor if a fight starts.  For offensive actions, such as direct action raids, I would probably choose Hornady’s new 190 SUB-X load to run with the SRD762Ti installed.  This load has a very low signature when used with a suppressor, and offers improved terminal ballistics over the typical 220 gr. SMK round.  Speaking of keeping a low profile, the Rattler fits well in the smallest backpacks and tennis racket cases.

The Rattler is a very mission specific platform that has changed the paradigm of the PDW as we know it.  It is compact, versatile, and reliable and is without any real competition in the industry today.  For those who choose to go the NFA route, and already have a registered M4 lower, the conversion kit is a great solution.  SIG also sells the full Rattler in a SBR model as well as the aforementioned pistol configuration.  However, the best option may be the route I took.  I can utilize the SBR in my home state and then install the upper on the pistol lower for travel outside the state.  Indeed, that seems to be the best of both worlds.

SIG M4 Rattler
SIG 220 gr. SMK888854863340.50″
SIG 125 gr. BHP169216621676300.60″
Hornady Black 110 gr. V-Max1912180618391060.05″
Hornady 190 gr. Sub-X916884895320.55″
Hornady 135 gr. FTX166816421656260.75″
Federal 110 SCHP166216441653180.80″
Federal 150 gr. Fusion14551447145180.80″
Velocity measured at 10 ft./Accuracy at 50 yds.  Aimpoint Comp M5 w/3X Magnifier


If you have not given the Rattler a look, I suggest you visit SIG’s pages:


***Shop GunsAmerica for your next Sig Rattler***

{ 18 comments… add one }
  • don e August 1, 2018, 12:24 am

    I would gladly pay twice the price for something that said jd smith on the box?

  • Steve Day July 31, 2018, 5:22 pm

    From reading the comments there appears to be a lot of “J D Jones” shills.

    I’d be interested in seeing the IP addresses of those posts to determine how many are from different (or non-TOR server) IPs.

  • Bill Christopher July 31, 2018, 4:41 pm

    Per Sig Sauer this morning, 7/31 the cost for the complete upper kit will be $1800.00 , and available for orders in the Fall..

    • Richard Stern August 1, 2018, 9:12 am

      Was that WITH CAN (as per the Sig person who never got back to me with P/N and pricing told me was the ONLY configuration available)?

      A that price – might as well the get entire firearm (which I’ve seen from $2K-$2,500 street prices), unless you already have a registered SBR lower or M-16 FA Lower you’re putting it on). The Sig lower is actually very pretty, ambi bolt/mag releases.

      Beware too on non-Sig lowers – the recoil system has a bad reputation for BREAKING AFTERMARKET TRIGGERS (or non-Sig). It comes back so fast and hard (in comparison to systems with buffer tubes, which are slower and a lot gentler on FCG’s).

      So much so – that Geiselle originally told me that their triggers were JUST FINE in my MPX (same lower in 9MM) and wren’t planning on MPX/MCX specific triggers – yet they now have an MCX trigger (with the metal shield) to keep from breaking disconnectors and pins (likely have stronger pins too). I broke 2 trigger pins in my Franklin Binary equipped MPX , until I replaced them with SS anti-walk pins (the kind that have screws instead of “spring grooves” – which is where the pins break – AT THE GROOVE).

      Not sure SIG is going to warranty anything on a non-Sig lower (especially import for guys running Class 3 Select-Fore Lowers). Also – THERE IS A DIFFERENCE between the recoil assembly (and likely Bolt Carrier) on a Full vs Semi configuration – so if you are planning on getting on of these for a registered C-III lower, you might want to verify whether the upper you’re getting is FA or Semi configured.

      Midwest Gun Works has a lot of parts for the MCX – including conversion uppers (though in 9 & 16″, not 5″ pistol) for the same price.

      This is not a cheap upgrade, nor a cheap firearm (is anything from Sig?). Nor a cheep shoot ammo-wise (in comparison to 5.56 or 9mm configurations).

      That notwithstanding – this setup gives me a woodie, and it’s on my list (behind the Tavor-7, TS-12, and Kalash USA AK). Still looking to do 2 more builds for the 2 Franklin Triggers I have on the shelf – I wanted one in 300BLK, but a short-stroke gas system, without a buffer tube (compact shooter) is so irresistible, I might have to wait until I can afford an MCX (or dump some of my custom build M-14’s).


  • Rob Garrett July 30, 2018, 2:15 pm

    Check out the SIG on-line store for the parts. The fixed folding stock retails for $150.00. The adaptor retails for $60.00.

    I apologize for not including this in the original article.

    In reference to Whisper vs. Blackout, there is not question that AAC took Blackout and changed it just enough to avoid having to pay royalties to JD Jones. The key to the success of BLK, over Whisper, is that AAC allowed it to enter the public domain and did not restrict ammo or firearm’s manufacturers from producing products. Jones would not do this with Whisper. It was certainly his right to do so, but it limited the number of companies that were willing to pay royalties for production.



  • Rob Garrett July 30, 2018, 1:55 pm

    Thank you for the comments and criticism. There is no doubt that AAC “reinvented” the Whisper. However, what made the 300 BLK a success is that AAC let everyone in the industry produce the cartridge without paying them royalties. JD has always required a royalty for anyone making firearms and ammo in Whisper. That is his right as he owns the rights to it.

  • Archangel July 30, 2018, 11:33 am

    The Blackout WAS OUTRIGHT THEFT of the 300 Whisper!
    BTW, the Blackout WIiLL chamber in the 5.56 and go BOOM!!
    However, the less known 300 Super is a slightly longer case with the same overall length so will not chamber in 5.56, does EVERYTHING the subsonic Whisper, er, Blackout will do and gives better supersonic performance than the 7.62×39!
    Why AAC did not rip that one off instead is beyond me.

  • Lyle Steve Bublitz July 30, 2018, 10:31 am

    Interesting article but gives credit to someone for developing the cartridge when JD Jones actually created it years before as the 300 Whisper. I would expect a little more depth of discussion and background than that.

  • Richard Stern July 30, 2018, 10:29 am

    I’m with Jaque. Is there a SKU for the complete upper kit? Or a parts listing to purchase separate?

    I’ve been looking to do a build for a pistol lower, with folding brace – already have the SIG folder attachment for my Franklin Binary equipped MPX – running an SB Collapsible Brace on that (for my cheek only of course).

    I have a couple of Franklins in the box, that I was looking to do a 7.62X39 & 300BLK builds on. A piston driven 300, with a (cough cough) “brace” – would be just the ticket.

    PART NUMBER (or numbers) to put one of these together, without having to purchase the entire firearm (I prefer to build my own lowers anyways).


    • Richard Stern July 30, 2018, 2:49 pm

      Took the initiative and called SIG this AM. Waiting on an email back with SKU and pricing.

      From what the rep relayed to me – the “kit” will be available WITH SUPPRESSOR ONLY – which should significantly INCREASES THE PRICE – plus you have to do a FORM-I for the can, whatever flaming hoops your particular jurisdiction AND wait for BATFE to get around to approving.

      As tacticool as this might be. I AM NOT PAYING $2,700 for an MCX (to avoid the BS of having to purchase with a CAN). It would probably cost ALMOST AS MUCH to assemble this FROM PARTS (though I will look into that angle).

      I can build a piston upper for 1/2 the price of an upper kit with can, and 1/4 the price of a whole MCX – and slap it on an in-house lower.

      Thanks for the TEASE though. LOL…


  • Rob July 30, 2018, 8:56 am

    I’m glad there are others calling him out for giving someone credit for work they didn’t do. Apparently they think if you use a different parent case and slightly change a dimension you get credit for creating a new cartridge. Props to Hornady for actually calling their cartridge 300 Whisper.

  • Carroll Hale III July 30, 2018, 8:55 am

    The 300 Whisper was used in IHMSA competition way before AAC came out with the 300 Blackout. Same case, same case dimensions, same bullets, same velocities… I’m wondering why JD Jones isn’t getting any credit (and royalties) for this.

  • Will July 30, 2018, 8:46 am

    So is SIG selling the conversion kits to the public?

  • Kent Williamson July 30, 2018, 6:37 am

    Amazing…NO comment on the 300 Whisper developed by JD Jones prior to 300BO… same chamber specs same bullet specs… an existing bullet design

  • Nick M July 30, 2018, 5:28 am

    Interesting how they say that is just how it (inaccuracy) is with the shift.

  • Tom July 30, 2018, 2:51 am

    I thought the 300 Blackout was based on the 300 Whisper by J D Jones who came up with that cartridge quite some time ago.

    • The Original Ralph July 30, 2018, 7:33 am

      the facts are that kevin copied the 300 Whisper, changed one of the Whisper’s dimensions by a couple ten thousandths (negligble) and called it the 300 Blackout. He saw an opportunity to take advantage of someone else work and a simple mistake SK Industries had made. SK had trademarked the term art “300 Whisper” and because of that, SAAMI would not accept the caliber for registration – it had to be freely licensed to the industry for SAAMI to accept it.

      Kevin, that that perpetual eye for how to take a shortcut and screw someone, saw the opportunity and took it. No real design accomplishment on his part, and no impetus from any “black” ops group wanting anything from him.

      just saying

  • Jaque July 30, 2018, 12:40 am

    Good article
    Missing is the most important information. Part numbers and prices.

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