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SIG Sauer M400 Rifle

The SIG Sauer M400 rifle is a direct gas-impingement AR rifle to satisfy traditional AR shooters.


With its Swiss-made 550 series, Schweizerische Industrie Gesellschaft (SIG) earned an impressive reputation for producing rifles used by a nation of riflemen. Now SIG Sauer makes the Model 556 and Model 551-A1 gas-piston operated rifles in Exeter, N.H., as well as the AR-based gas-piston Model 516 rifle. With so many piston-operated designs it came as a surprise when SIG Sauer introduced a direct gas impingement system AR-style variant, the M400 semi-automatic-only carbine.

Chambered in 5.56x45 mm NATO, the M400 has some distinct external and internal differences from most AR-style rifles. For sling options the lower receiver has quick attach/detach points machined on either side behind and below the forward assist push-button in addition to the more traditional sling swivels. There is also an ambidextrous magazine release with an extended swell on the left side to prevent it from being unintentionally depressed. The flared magazine well is machined with a more angular profile and features distinct vertical grooves on its front. The standard, left-side safety selector is set up for right-hand use, though the opposite side includes a raised tab, which allows experienced users to determine the position of the safety by touch on that side.

The upper and lower receivers are both forged 7075-T6 aluminum with a hard-coat anodized finish. The six-position collapsible stock rides on a commercial-size buffer tube, and the receiver extension nut is firmly staked to prevent it from coming loose. The buffer tube also includes a standard weight carbine buffer and spring. Inside the lower receiver at the rear a spring-loaded pin serves as a tensioning device, applying pressure between the upper and lower receivers. Tightening the fit between these parts reduces play and can lead to improved accuracy. Unlike some other tensioning devices, this one does not require the use of special tools for disassembly.

The commercial bolt and bolt carrier have a phosphate finish, and the gas key on the bolt carrier is properly staked. The chrome-lined and phosphate-finished 16-inch barrel is rifled with a 1:7-inch twist rate, which aids in stabilizing heavier bullets. The muzzle is topped with a standard A2-style flash hider. The barrel’s profile is slightly altered from that of a standard M4 in that it is missing the rear cut.

The sights include a correctly “F”-stamped elevation-adjustable front sight and a removable windage- and elevation-adjustable M16A2-type carry-handle rear sight. The upper receiver also has standard M1913 Picatinny rails with indexing marks to allow the user to more easily return an optic to the same place on the rail after removal.

At the barrel extension’s rear, the upper receiver includes an M4-style feed ramp, which is cut wider and lower than on a standard rifle to help ensure more reliable feeding from the magazine under adverse conditions. Inside the barrel extension, there is an extractor support pin which lines up with the extractor when the action is closed. In case of an over-pressure round or other cartridge failure, the extractor (as the weakest part of the bolt) could blow out, damaging the rifle and/or causing injury to the shooter. The extractor support pin was added to protect the shooter in the event of such a failure.

For the accuracy evaluation, we used two heavy hollow-point .223 Rem. loads, as well as a Winchester 5.56x45 mm NATO loading, fired from sandbags off the bench at 100 yards. The optic was a Trijicon TR20-2 AccuPoint 3-9X 40 mm scope with a Mil-Dot reticle that features an illuminated amber dot at its center. The accuracy results are shown the accompanying table. The single-stage trigger exhibited little creep and no stacking or overtravel, and it broke cleanly at 6 pounds, 8 ounces.

This lightweight compact carbine handled easily, and the oval polymer handguards’ double aluminum heat shields kept the shooter’s hands cool even through a series of rapid-fire strings. Several failures to feed were encountered using hollow-point ammunition, most typically upon initial feeding of the first round from the magazine. That said, full-metal-jacket ammunition functioned flawlessly in the accuracy test, as did a hodgepodge of several hundred miscellaneous 5.56x45 mm NATO ball rounds and partial boxes of full-metal-jacket loads leftover from previous tests.

With the addition of the M400, SIG Sauer is able to now satisfy all segments of demand for AR rifles, especially traditional gas-impingement enthusiasts. The improved safety features and other meticulous touches are a hallmark of this firm’s solid engineering.

Manufacturer: SIG Sauer, Inc.; (603) 772-2302; www.sigsauer.com
Mechanism Type: direct-impingement, gas-operated, semi-automatic center-fire rifle
Caliber: 5.56x45 mm NATO (.223 Rem.)
Receiver: forged 7075-T6 aluminum
Barrel Length: 16" chrome-lined
Rifling: six-groove, 1:7" RH twist
Weight: 7 lbs., 1 oz. (empty with removable rear sight)
Overall Length: 351⁄2" (stock extended), 32" (collapsed)
Magazine: 30-round detachable box (accepts standard AR/M16)
Trigger: single-stage; 6-lb., 8-oz. pull
Sights: elevation-adjustable front post, detachable carry handle mounted M16A2-type rear; M1913 Picatinny rail on upper receiver
Stock: six-position, black polymer collapsible: length of pull, 13½"; drop at heel, 1¾"; drop at comb, 2"
Accessories: plastic storage case, cable lock, owner’s manual, magazine
Suggested Retail Price: $1,099

SIG M400 Shooting Results

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24 Responses to SIG Sauer M400 Rifle

Brad wrote:
August 05, 2014

I have this rifle it is a mil-spec buffer. I have the Magnus mil-spec stock. A commercial stock is too loose. Also the buffer has dry film lube inside, commercial tubes dont. Not sure where this misleading info about the stock is coming from but its defiantly mil-spec

Bill Hall wrote:
November 08, 2013

Just bought one of these at the local Wal Mart, put 120 rounds thru it today. Super impressed and very happy with the gun. My Bushmaster is now for sale.

Rafael wrote:
September 18, 2013

Had her for a couple of months and the first time I took her to the range, was I impress and only getting good complements from everyone.

james wrote:
May 25, 2013

M400 buffer tube is mil-spec...

Steve wrote:
May 16, 2013

It is most certainly a mil-spec buffer tube. End of story.

Maddog150 wrote:
April 22, 2013

The stock tube on the M400 is not mil spec it is comercial. Milspec is thinner.

Deftone5 wrote:
March 27, 2013

Called Sig about my M400 to verify my Magpul CTS Mil-spec stock should fit and they claimed it should but it simply doesn't. Called Magpul and got some customer service reps voicemail who never returned my call.

Glenn wrote:
February 20, 2013

Just wandering what aftermarket stocks would work? Looking at the mako group gl-mag, would that work? I also am looking for a rail system to replace the standard heat guards. What length would work?

TJ106 wrote:
December 27, 2012

I have heard several military issue M16 make the sound you speak of. If the rifle is functioning normally meaning no issues with feeding. Clean/lube the tube & spring area, note some of Wilson Combats videos also talk about this sound in M4 style rifles & lube should help quiet things down.

k free wrote:
December 18, 2012

when shooting the m400 i hear a loud spring related noise in the stock..I am a wear of the action workings.But this is not a normal sound..It sounds like the spring is not in it's correct position.It's brand new..

Jeff Williams wrote:
December 08, 2012

I have Flattop M400 in NATO 5.56, no 223 stamp. 20in bbl Full size MagPul Stocks with cleaning kit butt compartment as well as a pistol grip compartment that holds 5 of your favorite rounds. The bbl is turned about 3/4 in at the end for a flash suppressor. Neither suppressor or sights included. The twist have changed it seems several times as maybe the buffer tubes The twist are Now carbine 1/7 the 20in is 1/8 the SIG 556 DMR 21in bbl is 1/9. I own the DMR 21" and 20" M400. These are not EVEN comparable to a BM, Colt no production rifle that is out there. MOA grps at 200y Mine is mil spec, but tight end of the tolerances. Upper lower play is non existent. Take apart the bolt it makes a sucking sound, it's that tight.Nothing in this rifle seems 'run of the mill'. I've never had any FTFeed FTF FTE. I've used 45g HP 55gRemington SP 62g NATO. The supplied 10 round mag, for bench shooting was problem, but a bit of filing or another mag solved it. I've used 10 20 30 round BM mags SIG mags they all worked. Some liked the bolt open to lock in tho.

Steve wrote:
December 07, 2012

If you are having trouble finding mags for the M400 Enhanced, my guess is u live in Illinois. lol I have 10 OD Green PMAGS and they are right on the shelf in most gun stores.

Karl wrote:
November 23, 2012

Have owned Colt, Bushmasters, Cmmg's Pwa's. As far asI am concerned, Sigs M400e beats them all, if Sigs customer service matched , in my opinion. It would be the perfect A.R

Farmer wrote:
November 14, 2012

I have the M400 enhanced and after spec'ing out the buffer extension it is standard military spec for an M4. As far as magazines, I have a ton of very beat up A-line, Colt, BMI and 'sterile' mags and have had ZERO weapon related mag issues. I personally use Magpul PMAG -Ws and love them. Superior quality, solid fit and a veteran owened company.

Dr Dave wrote:
October 27, 2012

I haven't had mag specific issues at all. Takes all the mags I've used in the Bushmaster, includ. aluminum 'green follower' type. The M4 feedramps seem to do the job just fine! I'm really happy with the Sig design, fit, finish & more accurate than Bushy H-bar. The tension pin holds the upper very tight to the Lower! Built in single point strap connector is great!

Scott M wrote:
October 23, 2012

MAGPUL mags work great. Midway has them by the truck load.

M Clark wrote:
October 01, 2012

It's a AR 15 any AR 15 or M16 Mag will work

Bill F wrote:
September 17, 2012

It is as above. Magazine: 30-round detachable box (accepts standard AR/M16)

Brian wrote:
September 03, 2012

Dave I have 2 M400 and the Magpul 20 and 30 round magazines work just fine.They're polymer,if you are actually looking for aluminum replacement's then I'm not sure.

Dave wrote:
August 28, 2012

Could someone PLEASE tell me why it is so difficult to find extra magazines for the M400?? I recently bought one and really like it. It seems like the only mags I can find are on the Sig Sauer site and they are only metal and 10 or 30 round. I am confused by other sites. Some say other mags Sig Saure mags will work on the 400, other sites most of the mags are for the other models only. Why? I am new to the AR...been shooting Garands. Easy to find magazines. Is it because the M400 is relatively new? Thanks.

Jim wrote:
August 19, 2012

The literature does say the m400e has a mil spec buffer tube. IDK, I will actually look.

SSM400E wrote:
July 31, 2012

I have the enhanced version in OD Green. It has a MIL-Spec buffer tube.

ccil wrote:
July 09, 2012

wrong--its comm size tube

Rowland Parks wrote:
April 22, 2012

Nice write-up, with one exception: The buffer tube is not commerical diameter. It is a 6-position mil-spec diameter tube. I thought this was important so people would not buy incorrectly sized aftermarket stocks.