Test Report: SIG M400 SDPC in .223 Remington

The SIG Sauer M400 model series stands for standard direct gas impingement AR-15 semi-automatic guns that are fully compatible with the platform. All M400s are U.S. manufactured in Exeter. The .223 Remington is the standard caliber, but several models are also produced in .300 BLK in the USA.

SIG Model 400 – Components

SIG M400 SDPC in .223 Remington broken down into its individual components
In technical terms, the SIG Sauer M400 is basically no different from other AR-15s – apart from the ambidextrous operability of the safety and magazine release.

In technical and dimensional terms, the SIG Sauer Series M400 is basically no different from any other run of the mill AR-15. The rotary cam bolt head gets its momentum directly via a gas tube over the barrel; the two-part receiver is built out of aluminum forgings.

SIG M 400 SDPC in .223 Remington with magazine
The special features of the M400 include the magazine catch on the left side of the housing and the sling swivel retainer above the pistol grip.

However, the M400 has some of its own features too. Among them is the ambidextrous operability of the safety and the magazine catch, including additional push button and additional retainers for a quick-release sling swivel studs on both sides of the lower receiver.

The lower receiver of the M400 is furnished with a small elastic wedge next to the rear take down pin, which lightly pushes apart the upper and lower receivers, preventing the receivers from shaking.

Extensive tolerances between the receivers of an AR-15 do not exert virtually any influence on the intrinsic accuracy of the firearm, but shaking can indeed irritate the shooter.

The SIG M400 SDPC in .223 Remington

The model’s full name is Super Dynamic Performance Rifle. The “C” in the model’s name stands for “carbon.” The carbon fiber composite, which is found on the hand guard. The name sums up SIG Sauer’s decision to tailor this variant to dynamic sporting disciplines, such as 3Gun or IPSC (open class). 

The barrel, made of stainless steel, measures up at 18 inches, a barrel length chosen for dynamic sporting variants. The octagonal forearm assembly is made by Lancer and enables free floating of the barrel at the lowest weight.

Apart from the many ventilation slots, the medium-length forearm assembly only offers a few mounting options. At the top, a short Picatinny rail can be found at the front, that is useful to mount a front fixed or foldable BUIS (Back Up Iron Sight), while a standard sling swivel stud is present on the lower side. A bipod can also be attached there as an alternative to the sling. 

Lots of accessories are usually not required for competitions such as the IPSC. At least not in terms of hand guards, whereby some shooters like to mount a small hand stop or barricade stop there.

SIG M400 SDPC in .223 Remington with hand guard
Lancer’s extra-light free floating hand guard features a Picatinny interface for a front sight base as well as a standard sling swivel stud on the bottom side.

With its width of 50 mm, the octagonal hand guard is basically no different from the various machined alloy forearm assemblies with the exception of the quadrail designs, but it reduces the overall weight and especially the front load of the weapon. 

The SDPC only weighs just over three kilograms, despite its relatively long barrel that isn’t too thin either with a diameter of 18 mm. Some other models of the M400 series additionally offer carbon fiber rear stocks, but the SDPC instead comes with a collapsible polymer stock produced by Magpul that is lockable in six positions. 

The Magpul CTR rear stock can be fixed into the selected extension length via the external rocker button so that the assembly does not shake on the buffer tube, screwed into the rear part of the lower receiver and also to facilitate a steady hold of the supporting hand in the prone position. The locking mechanism is released again by pressing on the rocker in the inside of the stock assembly. 

The enlarged trigger guard is also made by Magpul. The slip-resistant, roughened soft rubber pistol grip, on the other hand, is from the American grip specialist Hogue.

SIG M400 SDPC in .223 Remington, muzzle from the front
The 62 mm long comp by JP Enterprises significantly reduces the muzzle flip. The diameter of the stainless compensator amounts to 25 mm.

There is much to like about the workmanship of the present design. The housing, the bolt group and barrel all have a cleanly processed design. The accu-wedge type in the M400 helped to conceal the slight play between the receivers here. Both retaining screws of the gas intake have been strenuously caulked in the factory, so that the shooter need not worry about the slightest looseness when it comes to the most important functioning. 

The castle nut of the buffer tube has also been caulked with the lower receiver end plate, so that the buffer tube itself cannot come loose – such detailed attention is simply not seen with other AR-15s offered on the European civilian market.

SIG Sauer has chosen a two-stage trigger manufactured by Geissele on the M400 SDPC. With the test copy, the trigger released approx. three millimeters dry with just over two kilograms of trigger weight after a short creep, falling through only minimally.

The ten-round magazine of the AWM series from Lancer also left a high-quality, sophisticated impression. Its body largely consists of transparent plastic, the upper area including the magazine feed lips, in contrast, is made of sheet steel.

Experiences: SIG M400 SDPC in .223 Remington on the range

SIG M400 SDPC in .223 Remington side view from the right
Both the muzzle attachment as well as the carbon fiber forearm assembly give the SIG Sauer M400 SDPC its distinctive appearance, here with Kahles 6-24×56.

Equipped with a Kahles 624i, which can be magnified up to 24×, on a Milmont mount from MAK, the 100 meter range was the target. In the field of precision, the Lapua Scenar match cartridge took the lead with 21 millimeters. 

The dry releasing Geissele trigger has a competitive edge compared to standard triggers. The compact compensator from JP Enterprises proved to be very effective in the shot despite its rather small dimensions and successfully suppressed the recoil and the muzzle climb. 

The small compensator cannot completely conceal the already gentle muzzle climb of the .223 Remington, but the muzzle rises only minimally especially when using lighter bullets, much less than with an uncompensated AR-15. 

SIG M400 SDPC in .223 Remington muzzle technology
The small pressure pin in the lower receiver braces the housing halves on the SIG M400, so that no disrupting shaking occurs.

With the 50 grains load of the Norma Varmint variant, the test copy did not want to repeat this at all. The weapon only functioned here in manual mode. 

However, this could also be attributed to the type of ammunition itself or to the actual production batch. Since the 50 grains variant of the V-Max cartridge from the Swedish manufacturer was initially tested with a semi-automatic unit. All other loads ran like a charm in the M400 SDPC.

SIG M400 SDPC in .223 Remington – the Conclusion

The SIG Sauer M400 SDPC semiauto rifle proved to be a very pleasantly designed device and was thoroughly convincing with regards to its workmanship and in terms of its suitability for shooting sports. 

The extra-light hand guard and the compensator also gave the impression of being very practical in the intended field of usage. Sure enough, a somewhat longer version of the carbon fiber handguard would be more desirable in the view of some testers.

More information about the SIG M400 SDPC in .223 Remington can be found directly on the website of the manufacturer.

A further test of the SIG Sauer self-loading rifle can be found here:

Test: SIG Sauer 716 Patrol self-loading rifle in .308 Winchester.

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