SIG Sauer M400-SDI X-Series Rifle: SIG's Best AR-15 Yet?

Loaded with extra ambi controls and features, the SIG Suaer M400-SDI X-Series doesn't require any upgrades and is ready to go out of the box.

April 08, 2024 By Oleg Volk

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Sometimes a rifle just works. Fresh out of the box, it runs 100% reliably, shoots accurately, handles comfortably, and basically gives no trouble at all. You zero it, aim, and hit the intended target over and over again. It’s a very boring rifle, from the perspective of a writer. Or a very exciting one from the perspective of the marksman. In any case, that rifle is a SIG Sauer M400 SDI X series. I actually lied about this rifle being perfect. It has one minor imperfection, but I will mention it in due time. Out of the box, it’s a 16-inch, direct impingement carbine with a lightweight free-floated rail and a collapsible Magpul SL stock. This particular gun came with a SIG Romeo 4SXT-PRO red dot, a very capable closed emitted 1x20mm NVG- compatible optic with selectable Quad reticle, and a 30-round Magpul magazine. Just about the only missing accessory is the sling and a QD receptacle for it.

sig-sauer-m400-sdi-x-series-02 — Well-chamfered edges of the SIG M400 forend are easy on the shooter’s hands.

The first thing I noticed were the fully ambidextrous controls. Having recently handled a left-ejecting upper on a standard right-hand lower, I can appreciate how this makes the M400 a much easier rifle to handle for left-handed shooters. In right-handed use, I found the bolt release lever helpful for shooting from sandbags: I could load the magazine but keep the bolt open until the rifle was aimed again, both to maximize barrel cooling airflow and to keep the muzzle pointed in the safest possible direction once the gun returned to battery.

sig-sauer-m400-sdi-x-series-03 — The flash hider/ compensator (top right) is slightly longer than the military A2, and seems to be more effective. Ambi charging handle (bottom right) is easy to reach even under overhanging scopes. Simple and easily adjustable, Magpul SL stock proved very comfortable. Without finger grooves, SIG pistol grip fits various size hands well.

The implementation of the streamlined controls was done well, with the off-size controls not interfering with handling. In effect, SIG implemented some of MCX-SPEAR features in a more affordable product line. Similarly, an ambidextrous 90-degree safety lever is short enough to not impinge on the shooter’s thumb, but long and wide enough to be easily operated. Both receiver halves are extensively machined, with proper fencing for controls against accidental activation, and with smoothly radiused edges against skin abrasion.

sig-sauer-m400-sdi-x-series-04 — The SIG Sauer M400 SDI feels good in hand.

Curiously, the flats around the safety levers retain visible machining marks that are absent everywhere else: they look more like a fashion statement than an oversight, given the smooth and clean finish everywhere else on the receiver. The fit between the upper and the lower is excellent. The charging handle is likewise ambi, and can be activated from either side or from both at once. While ejection is right-side only, the empties come out on a far enough forward angle to clear the shooter in any stance, even if bladed from the left shoulder. The free-floating forend is thin enough for a comfortable hold, wide enough to provide adequate separation of the hand from the hot barrel. Standard weight barrel is threaded 1/2x28, and provided with an effective closed-bottom cage flash hider augmented with small compensator openings at the top. The comp isn’t concussive but does its job keeping the muzzle down.

sig-sauer-m400-sdi-x-series-05 — Greatly multi-featured SIG Sauer Romeo 4SXT-PRO red dot is a fitting companion sight for this rifle. Original design ambi controls, including the right-side bolt release lever, are very well implemented. Safety levers and left-side magazine release are easily accessible without getting in the way during actual shooting. All SIG rifles are made in New Hampshire, one of the best states for 2A.

Romeo 4XT-PRO red dot offers four reticles: 2MOA dot, 2MOA dot inside 65MOA circle, and the same two with the addition of three progressively smaller BDC dots. Running on a single AAA battery, this is a lightweight and streamlined sight well-suited to defensive use of the M400. It comes with integrated front and rear caps, and Killflash attachment. Waterproof and rated to 50,000 hours on a battery, it also turns off the reticle after two minutes of immobility, snapping back on at any motion of the rifle. While this is the sight to have for defense, I wanted more magnification for testing accuracy.

sig-sauer-m400-sdi-x-series-06 — Vudu 1-6x FFP scope allows accurate ranging using the reticle at any magnification. The SIG Sauer romeo4XT-Pro has four reticle options.

The EOTech Vudu 1-6x FFP scope is one of the few to do a good job with a first focal plane reticle in an LPVO. It is water-resistant, with membrane switches for illumination on/off and brightness settings. Gray finish reduces heating up in the sun. A 65MOA outer circle similar to 4XT-PRO’s works well for rapid acquisition at low or no magnification. The faint central crosshairs work for finer aiming in good light. Past 2x magnification, the center crosshairs become the main aiming mechanism, and at 4x MRAD subtensions on the crosshair become clearly visible. Ranging and holdovers work the same at all magnifications. 6x maximum magnification is reasonable up to the maximum effective range of 5.56mm from a 16-inch barrel.

sig-sauer-m400-sdi-x-series-07 — Although endowed with extra ambi controls, M400 SDI field-strips like any other AR15. The upper to lower fit is tight and even.

Zeroing this rifle took just three rounds, the first one at 10 yards to roughly center the reticle, another at 60 to verify, and a third to click the necessary adjustments to ensure 100 yard zero. That’s the joy of working with an accurate rifle. Felt recoil, as is usual for an AR15, was notable only by its absence. The textured rubber recoil pad is there more for a secure connection to the shoulder than for recoil reduction. This is where my sole complaint with the rifle comes in: the trigger weight. It felt heavier than expected, and my analog gauge measured it between 4.5 and seven pounds, depending on how quickly I pulled. As a result, my first few groups had excessive horizontal dispersion, after which I remembered Appleseed riflemen workshops and got accurate.

sig-sauer-m400-sdi-x-series-08 — Hornady Frontier BTHP 69-grain Match (bottom left). Hornady Frontier 55-grain FMJ (bottom right). Corbon 62-grain T-DPX (top right). Federal Matchking 69-grain BTHP (top left).

The ergonomic pistol grip with a high beavertail made it easy. Apparently, SIG anticipated my concern, as their website now lists a flat blade adjustable match DH3 trigger as standard for the rifle, rather than the more basic two-stage version I have. The targets speak for themselves. Unfortunately, I had to shoot them from 60 yards as my 100-yard position was not accessible. Given the fast 1:7-barrel twist, heavier bullets may perform even better, but I would prefer to try that once I lighten the trigger — the current limit on accuracy is probably my own skill set. Excellent performance with match ammunition is no surprise, but 1.5 MOA groups with defensive ammunition are a big plus for this rifle, considering the likely use on medium game like pigs. Defensive fights tend to happen close enough that mechanical accuracy matters less. Good accuracy is, however, an indicator that everything in this rifle comes together consistently, with no need to “work in surfaces” as is the case with lower-quality guns. For defensive use, I ran the rifle off-hand with the scope set to 1x and 2x: target acquisition was rapid, accuracy sufficient to get torso hits out to 75 yards.

sig-sauer-m400-sdi-x-series-09 — Minimal recoil allows safe firing even from unorthodox positions.

What else does this rifle need before it can be deployed? I would add a light/laser combination: it can be placed far enough forward to avoid beam occlusion by the muzzle. A laser can be a useful secondary aiming method without raising the rifle to eye level. Other than that, range time and a dope chart. One reason why I used an MRAD scope rather than one with pre-calculated BDC reticle is the barrel length: 16 inches produces lower velocity than the test barrel for which most scopes are calibrated. At 300 yards, the discrepancy is small, by 400 it amounts to several inches of extra drop. For example, with 100 yard zero, 500 yard hold would be half-way between the 3rd and the 4th reticle hash marks. While different bullet weights vary slightly in points of impact, this setting is for Federal Gold Match 69 grain, a more sensible load for distance than something this rifle shoots less accurately.

SIG barrel is 4130 chromoly steel, very durable: this rifle can be expected to keep its accuracy for a long time. Combining excellent ambi interface and thus-far perfect reliability, The SIG Sauer M400 SDI has handily won a spot on the range favorites rack. Using it, I can concentrate on practicing tactics and marksmanship without distractions of function failures or poor accuracy. Rifles that feel good in hand usually shoot better, and this M400 SDI definitely fits me right and performs above reasonable expectations.