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Sig Sauer M400 Carbine


firearms have earned a reputation over the years for extreme reliability. Its designs have also been known for advancing the technology and manufacture of modern weaponry. The P210 9mm pistol is considered one of the most inherently accurate combat pistols every made.

The innovative Sig P220 pistol had its muzzle welded to its sheet metal stamped slide and this cost saving process maintained the gun’s structural integrity and accuracy. The recently introduced Sig P250 handgun offers true ground-breaking modularity, built around its serial numbered chassis, with an array of interchangeable grip frames, slide lengths and caliber choices.

SigSauer, located in Exeter, New Hampshire, recently introduced its new M400 carbine. While it may appear similar to their 516 rifle, the M400 utilizes the time proven direct gas impingement action. The 516 uses a gas piston action that keeps hot gas and fouling out of the rifle’s action. Piston operated AR-15 style rifles have become more common and are generally seen as an improvement on the original gas impingement action originated in the 1960s.

One may reason that the M400 5.56mm (.223 Rem) carbine appears at first glance to be a throwback to the gas impingement action first introduced more than four decades ago. When everyone else is converting their impingement system to a piston system, why would Sig take an apparent step backwards? There’s a simple answer to that query: American law enforcement asked for it.

Every police officer would like to utilize the latest avant-garde equipment, but realistic police department budgets drag our fantasies back down to earth. Stockpiles of Mil-Spec, wear and tear replacement parts (firing pins, gas rings, springs, pins) can be employed on most brands of AR-15 style rifles. SigSauer was approached by several large police departments and asked to bring forth a gas impingement version of the Sig 516 gas piston rifle that could make use of all the replacement parts stored in their armories.

Sig realized the demand for a less expensive version of their 516 rifle and met the request with the new M400. The new gun operates with the proven gas impingement system that does get dirty, but functions well when properly lubricated. The M400 is made in the USA and offered in two versions.

Classic and Enhanced Versions

Both Classic and Enhanced editions of the M400 are outfitted with a 16-inch, chrome lined and phosphate coated, 1 in 7-inch twist (for bullets longer and heavier than 55 grains), cold hammer forged barrel. An M16A2 flash suppressor adds another inch to the barrel, resulting in an overall carbine length of 39.5 inches for the Classic and 35.6 inches for the Enhanced (both with stocks fully extended).

The overall length of the M400 allows it to be stored vertically inside a squad car with its stock fully compacted. The direct gas impingement operating system features the proven rotating locking bolt. The Classic and Enhanced carbines weigh 6.5 and 6.7 pounds unloaded, respectively.

The Classic M400 has a round carbine handguard, plain hard plastic grip, and M4 style collapsible stock. The fixed front sight base (with bayonet lug) has an adjustable post for elevation and is matched up to a detachable carry handle with a built in rear sight. The dual aperture rear sight is adjustable for windage. Magpul supplies the M400’s excellent 30-round, impact resistant, polymer PMAG with pop off dust cover, but the rifle will fire with virtually any standard AR-15/M16/M4 5.56mm magazine.

The Enhanced M400 we used for testing adds a Magpul MOE® handguard, grip, and 6-position collapsible stock. The forearm’s triangular shape and smooth surface is very comfortable when shooting. Slight ridges provide for a secure grip and a forward lip provides for the ability to use the offhand index finger to further pull the stock into the shoulder. Slots allow mounting of rail sections at 2, 10, and 6 o’clock positions for accessories such as lights, lasers, and vertical grips.

The pistol grip is textured and built up at the rear where it follows the contour of the lower receiver for excellent trigger control. The grip is also hollow for battery or other storage. The stock’s lever that controls its adjustment is protected against accidental release by a triangular enclosure and has a 1/4-inch rubber pad for recoil management. The stock’s smooth, slightly angled contour provides a solid and comfortable cheek weld that naturally lines up with the sights when shooting.

The Enhanced M400 includes a rear sight made by Sig that attaches to the top Picatinny rail. The dual aperture rear sight is windage adjustable and flips down with a solid detent (but does not lock) for clearance when an optic is mounted on the rail.

Ambidextrous Controls

Both Classic and Enhanced versions of the M400 have left and right side push button sling swivel attachment points as part of the lower frame. The holes can be used in conjunction with the forward sling attachment point or for fastening the popular single point sling.

The safety selector on the test rifle was only on the left side of the receiver, but Sig plans on an ambidextrous switch for newly produced carbines. The switch rotates smoothly and secures with a positive click. The magazine release button is ambidextrous, which is a terrific aid for left handed officers (and for those training them). The bolt release is in its standard position on the left side but the fencing around the ambidextrous magazine button prevents the addition of a Magpul Battery Assist Device.

While the M400 is budget priced, it does include M4 extended feedramps (matched from the barrel to the frame to allow smooth cartridge feeding from either side of the magazine follower), a staked castle nut and staked gas keys (to prevent loosening when the gun is fired), and a Mil-Spec buffer tube. These are usually the first features eliminated when a manufacture needs to reduce cost. Sig kept these desirable attributes.

Budget Price, but Not Cheap

Everything about the Sig M400 feels right when you shoulder the weapon. The carbine’s outstanding balance feels light when sighting on target. SigSauer was wise to furnish the Enhanced M400 with Magpul polymer hardware. Magpul is a leader in the AR-15 accessory arena. Their products are well thought out and tested and the result is that they work very well.

The pistol grip fills in the area behind the frame and feels ideal in your hand due to its angle and circumference. The grip’s front and rear grip serrations work well with the non-slip stippled sides. The forearm also feels just right and its smooth polymer surfaces sure beat the sharp edges of quad aluminum rails. Rail covers can become bulky when covering three sides of a quad rail. The M400’s forward gripping area is relatively thin and naturally curved for your support hand.

The collapsible stock’s release lever is protected within a triangle against accidental activation and intuitive in operation. The rubberized butt pad has non-slip ridges and helps to absorb the slight recoil of the 5.56 NATO round.

At the Range

The M400 was totally reliable at the range, keeping with SigSauer’s reputation. The iron sights were very usable at 50 and 100 yards for consistent hits on human sized targets. The trigger has very little take-up and breaks cleanly at 5 pounds, 10 ounces on my Lyman digital trigger pull gauge. There is no over-travel after the hammer falls. The trigger is just about perfect for a patrol rifle.

The quality trigger helped the M400 shoot very favorable groups at 100 yards when topped with a Leupold 3X-9X scope in a Burris PEPR (Proper Eye Position Ready) mount. This mount provides perfect eye relief on AR-15 rifles with top Picatinny rails and its lever actuated, quick detach ability returns to zero every time.

Off a sandbag, during a cold and windy day, I fired a 10-shot, 100-yard group with the M400 using Remington 55 grain FMJ 5.56mm ammunition. I placed seven rounds in the 10-ring and three into the 9-ring. Six of the ten bullets made for a 1.3 inch group. I am going to call that MOA-level accuracy a factory patrol carbine. This Sig is not designed for sniper use, but its accuracy is very close to that standard of accuracy.

Shooting the M400 at close to medium distances with the iron sights or an electronic red dot resulted in guaranteed hits every time. Utilizing a magnified optic to strike the 10-ring at 100 yards proved to be pretty much a sure thing. Everything about the rifle worked well.

The Right Gun at the Right Time

While gas piston rifles may be the cutting edge of technology and they keep hot and dirty gas out of the chamber area, they are also heavier and more costly. The original direct gas impingement system as used on the AR-15 style rifle is time tested, reliable, lightweight, and less costly. That said, the Sig M400 5.56 carbine isn’t a throwback, but rather it appears to be the right gun at the right time. The Classic M400 carbine has a $1,099 MSRP, while the Enhanced model is $1,120.

The SigSauer M400 is made in the USA, rugged, reliable, and packed with quality components. A theft report may have to be filed because the M400’s price makes it a steal.

Steve Tracy is a 22-year police veteran with 20 years of experience as a firearms instructor. He is also an instructor for tactical rifles, use of force, less-than-lethal force and scenario-based training. He can be reached at

Published in Law and Order, Jan 2012

Rating : 9.1

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Posted on : Mar 27 at 9:57 AM By Bill

from my understanding Sig does not use cold hammer forged barrels like your article states. If they do, I'd like to know where you got your information as I've been over this issue with many folks and Sig many many times.


Posted on : Jul 27 at 3:24 PM By Michael G

Great article !! Very clear , and concise and yet short. Great job Steve Tracy.

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