SIG M400 Direct Gas Impingement Carbines

Steve Johnson
by Steve Johnson

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
Caliber5.56m NATO
Capacity30 rounds
Barrel16″ M4 profile
Overall Length34.6″
Weight5.65 lbs
Front SightPost
Rear SightCarry Handle
Trigger pull7.6 lbs
MSRP (Price)$1065 (standard model)
AvailabilityShipping now
Steve Johnson
Steve Johnson

I founded TFB in 2007 and over 10 years worked tirelessly, with the help of my team, to build it up into the largest gun blog online. I retired as Editor in Chief in 2017. During my decade at TFB I was fortunate to work with the most amazing talented writers and genuinely good people!

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2 of 19 comments
  • W W on Sep 30, 2011

    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.

  • Chad Housman Chad Housman on May 19, 2013

    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?