SIG M400 Direct Gas Impingement Carbines

Because of popular demand Sig Sauer has developed a line of Direct Gas Impingement AR-15s to complement their line of SIG 516 piston-operated ARs.

SIG M400

The SIG M400 features a SIG 516 lower receiver with it ambidextrous magazine release, rear quick-detach sling mount and tensioning device that ensures a tight lockup between the lower and upper receivers. The upper is a standard M4-style upper receiver. The enhanced model features Magpul OEM stock, pistol grip and handguard.

SIG M400 Enhanced
Caliber 5.56m NATO
Capacity 30 rounds
Barrel 16″ M4 profile
Twist 1:7
Overall Length 34.6″
Weight 5.65 lbs
Front Sight Post
Rear Sight Carry Handle
Trigger pull 7.6 lbs
MSRP (Price) $1065 (standard model)
Availability Shipping now

Steve Johnson

Founder and Dictator-In-Chief of TFB. A passionate gun owner, a shooting enthusiast and totally tacti-uncool. Favorite first date location: any gun range. Steve can be contacted here.


  • erwos

    So… while I admit to being one of those rare guys who thinks DI is superior to piston, who the hell is the market for this rifle? You can buy a Spike’s for $850, and it’s real hard to imagine that this is better quality. Hell, a Daniel Defense gun is only a couple hundred bucks more, which is basically the cost of the rail system.

  • justin

    I have have the 516 and if this rifle is as well made as the 516, it should do well. No wobble between the upper and lower receivers and tight tolerances in the gap between the 2 halves. But the price is a bit high, I picked up my 516 Patrol for only 200 bucks more than that MSRP. But I do love that MOE handguard on DI ARs. I nearly bought the S&W M&P15 with the MOE gear instead. Compared to a railed forearm that MOE forearm makes for a light rifle.

  • Sian

    I’d about be sold if it was a mid-length. I don’t understand why all these 16″ uppers are carbine length when a mid is pretty much superior in every way except accessory availability.

  • Freiheit

    OMG OMG OMG another AR-15!

  • H

    Pardon the simple question, but isn’t the whole piston element an improvement from DI? What are the benefits to DI over a piston?

  • Erik

    I am interested in one of those lowers-I have no use for an M4ish upper though. Hopefully Sig will make just the lowers available for a reasonable price.

  • Andy

    It’s only a matter of time before everyone hops on the AR bandwagon. I love reading the descriptions of how each of these new ARs is groundbreaking and different from every other AR out there.

  • a_is_a


  • Samopal

    Would you happen to know if SIG Sauer is making these uppers, or if they’re just slapping someone else’s on their lowers and selling them?

    Also what’s with the AR’s industry’s love for carbine-length gas systems? I’m no AR expert but it doesn’t seem to make any sense with a 16″ barrel. We have mid-length systems now…why not use them?

  • Lance

    When there will be 20ich A2 or A4 uppers??? Looks nice though too expensive though.

  • Lance

    I do like the Magpul accessories on the premium model though.

  • jdun1911


    DI AR have a piston. What they don’t have is a physical op-rod. They really have no real advantage only perceived advantages.


    My carbine system work well with my 16″ barrel. Why change when there are more options for carbine gas system than mid-length?

    I pretty sure Sig assemble their own uppers. If you have the right tools, assembling uppers is easy and fast.

  • drewogatory

    Where are the darn 20″ 1/12 twist A1 pencil barrels or uppers? That’s what I want, 55 gr. is cheap and readily available and the .22 conversions shoot alot better in 1/12.

    • jdun1911

      Build your own upper. That’s what I did. Pretty simple if you have the right tools.

  • PLUS

    What does “complement” means?

    Why don’t they keep manufacture SIG 516?

    Sorry, I haven’t update my info of SIG’s AR type carbine for a long time, can anyone tell me about this?

    • PLUS, what I mean is that it increases the appeal of SIG rifles. The SIG 516 is still the main SIG AR-15, but for those people who prefer DI over piston operation, they can now buy the SIG M400.

  • Craig_PHX

    Another thumbs up for gas impingement over piston guns!

  • Aurelien

    Well, one thing nobody seems to see is that SIG is simply following the plan of Remington : they get a piston-operated design out, but keep a DI design on the side.
    And the reason why is military contracts. In the likely event the US troops never get a new gun platform or upgrade kit, SIG can just get a contract to manufacture cheap, light DI M4A1s.

  • W

    I wouldn’t necessarily agree that this is a plus over gas piston guns, though i am delighted to see SIG produce a DI AR15. I don’t have a particular problem with DI, though recognize the advantages of gas piston technology (which has a disadvantage of lacking a common standard due among different companies).

    In response to fulton armory,

    “Can’t free float, at least to not anything like the same extent as direct impingement.”

    given the technology that exists, such as the cold hammer forged barrel, polygonal rifling, or chrome-molybdenum barrels, this problem can be remedied. Gas piston carbines can be highly accurate, and as combat rifles/carbines, sub-MOA accuracy is a rather excessive goal, even with a 20″ designated marksman rifle. If you want effectiveness beyond 500 meters, it is wisest to change your caliber from the 5.56mm. The limitations in accuracy are placed on the caliber itself, not the “disadvantages” of a gas piston on a 5.56mm platform.


    The PWS mk 1 carbine (just as a example) weighs 6 lbs and the HK 416 weighs 7.5 lbs give or take. In comparison a Colt M4A1 weighs 6.3 lbs empty. So the difference varies between manufacturers of gas piston AR15s, though, by analysis, direct impingement carbines can be marginally lighter (1-2 lbs lighter is not a game changer).

    “More complex, more components to foul, corrode or go bad.”

    I wouldn’t agree that there are “more components to foul, corrode, or go bad”. With a gas piston, the fouling and corrosion is focused on the piston system and the gas key. with a DI gun, it is on the gas tube, bolt, and bolt assembly. Both components are invaluable for the operation of the gun, so its a manner of picking your poison.

    “Much more costly”

    gas piston may be more costly simply because it has not been adopted in substantial numbers by a military, though this may change in the future. Millions of DI guns have been manufactured. Since gas piston carbines have only seen widespread service after the mid 2000’s, it is only a matter of time before prices are affected by the number of guns produced.

    “No standard, every system is different, heck, some vendors have changed designs two or three times! The owner is locked into a particular vendor for support, and that’s assuming that the vendor will continue to support early iterations. If not, you have a thousand dollar tomato stake.”

    this i cannot disagree with. this is the glaring disadvantage of the gas piston system unfortunately.

    I believe the gas piston system has more advantages to those using SBR’s, suppressors, different ammunition types, and automatic fire, with the best example being USSOCOM (which is why they use the HK 416 largely). Civilian shooters perhaps may be less concerned with using a direct impingement rifle.

  • I’m having some higher gas pressure release on ejecting cases. After the rifle is fired, it leaves dents in the cases. Can I adjust the gas block?