TFB Review: The Full-Featured SIG M400 SDI XSERIES


SIG Sauer has really made a concerted effort over the last decade or so to bring functional, practical, and sometimes even reasonably priced firearms and equipment to the market that cater to more than just military operators. One particular turning point that really indicated to me that SIG was adapting its product lineup for modern firearms audiences was when it released the P322 – probably one of my favorite firearms to be released in the last decade. This also leads to SIG’s M400 Series, their own turn-key offering to the DI AR-15 market. There are several different versions of the M400 each of which caters to slightly different firearms consumers ranging from those on a tight budget to those with hunting or even 3-Gun competition in mind. Our review subject of today is another sibling from the M400 family, the M400 SDI XSERIES. This specific M400 offering is SIG’s answer to those who want a bit more than the average budget AR-15 offers including some design cues taken from the SIG MCX-SPEAR platform.

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TFB Review: The Full-Featured SIG M400 SDI XSERIES

SIG Sauer recently sent out the M400 SDI XSERIES for review and included one of their more budget-friendly LPVOs, the SIG TANGO-MSR 1-10×28 SFP. As is fairly common here at TFB, fellow contributor Hop received an identical copy of the M400 SDI XSERIES and as a result, I’m able to draw testing results and overall performance characteristics from two different copies of the same rifle. So what exactly does the SDI XSERIES M400 offer you and how much is it going to set you back?

TFB Review: The Full-Featured SIG M400 SDI XSERIES

Some features that make the M400 SDI XSERIES stand out are its hybrid flash hider/3-chamber compensator, 2-stage match lite trigger, lightened upper and lower receiver set, a unique handguard clamp system, and fully ambidextrous controls nearly identical to those found on the SIG MCX-SPEAR.

TFB Review: The Full-Featured SIG M400 SDI XSERIES

All of these features may or may not sound appealing to you depending on what you’re looking for in a rifle or how much you’re willing to spend for these features. Some of the more “boring” details of the rifle are similar to just about any other AR-15 on the market and this includes its anodized finishes, chrome/moly steel 1:7 twist barrel, a mid-length DI gas system, and Magpul SL stock. All the extra bells and whistles that the M400 SDI XSERIES has only make the rifle about $1,300 from most online SIG dealers. Not quite budget territory but also not nearly as expensive as some comparable fully ambidextrous control rifles on the market.


The SIG M400 SDI XSERIES rifles that both Hop and I had performed flawlessly in the reliability department. We collectively ran almost every off-the-shelf ammunition type we could find including various flavors of PMC, Wolf, Winchester, Hornady, some of my custom handloads, and even some Australian match ammunition during our accuracy testing sessions. Each type of ammunition ran well through the rifle in terms of feeding, cycling, and ejection.

TFB Review: The Full-Featured SIG M400 SDI XSERIES

I took the liberty (as I often do with SIG products) of attempting to abuse the rifle as much as possible. By abuse, I don’t mean outright trying to damage the rifle, but rather using and caring for the rifle in such a way that it’ll test some of the extremes the rifle might experience during its service life. This included several wasteful bump-fire mag dumps, some spills in snow, several range trips in heavy rain, and of course each of those wet range trips ending with me promptly stuffing the rifle, and its attached TANGO MSR optic in a gun case to marinate for a bit.

Throughout all of this abuse, the rifle has remained very reliable and hasn’t shown any signs of slowing down. After several range sessions of reliability testing, both Hop and I took our rifles to different two-gun competitions where we both had great results in terms of reliability with our rifles. My optic, for all its faults that I will explain here in a minute, continued to function and didn’t fog up or lose its zero during any of my live-fire testing sessions, nor did it lose its zero after being flipped 4 times inside of a UHAUL trailer across the Wyoming tundra.


While I kept the included 1-10x28mm SFP TANGO MSR riflescope mounted on mine, Hop opted to replace his LPVO with a Trijicon G33 magnifier and a ROMEO4 XT PRO. While the latter seems like the much more practical option to me, the 1-10 LPVO isn’t without its merits, but I think the 1-10x28mm TANGO MSR might be the wrong optic for this rifle. Primarily because I think the M400 SDI XSERIES rifle is punching above the TANGO MSRs weight class.

The TANGO MSR 1-10x28mm SFP Riflescope

The TANGO MSR is, in my opinion, attempting to bite off more than it can chew. While the thought of a 1-10x28mm LVPO sounds great on paper, in reality, the optic is really only useful in a select portion of that magnification range unless you’re planning on shooting the rifle only from a bench. The TANGO-MSR features 11 levels of selectable reticle brightness, capped windage, and elevation turrets, and comes complete with spring-loaded objective and ocular lens caps. The mount that the TANGO-MSR comes with is nearly identical to the one that comes on the TANGO6T, which includes pre-marked torque specifications. Lastly, the TANGO-MSR 1-10 comes in at an all-inclusive price of just $600 from most online retailers, it’s quite affordable for an optic with a good feature set.

Above are all things I really like about the TANGO MSR but these positives come along with some pretty serious negatives. First off is the magnification range. As I mentioned earlier, the 1-10 magnification range of the TANGO-MSR seems nice until you try to use it on a magnification setting above 6 and aren’t shooting from a bench. About 40% of the upper magnification range is next to impossible to maintain a proper sight picture with during sustained strings of fire and this fact reared its head most clearly when I was attempting to take shots out to 300 yards during a competition.

I found it extremely difficult to maintain a sight picture on the higher magnification settings which forced me to use the lower settings (1x-4x). This decreased my ability to properly see and engage some smaller targets that I would have liked to have a better sight picture on. To put it plainly, while you theoretically could use the higher magnification range of the optic, in practice, it’s just too difficult with the exit pupil and eye box that the TANGO-MSR 1-10 provides. However, for its price range, this might be forgivable and to some people, specifically, hunters who will most likely be shooting from a stable location, these complaints I have may not matter as much.

Mechanical Accuracy

Despite everything I had to say about the TANGO-MSR 1-10, the rifle itself is quite accurate and if I hadn’t bothered to swap out the MSR 1-10 for a Leupold Mark 5 HD 5-25×56, I would have never discovered just how accurate the rifle could indeed be. For this rigorous accuracy test, Hop, and fellow writer Lucas D both joined me in putting groups on paper at 100 yards using this setup and we had some pretty good results with match ammunition that put the rifle close to 1-MOA in the accuracy department – using match ammunition. The Match ADI Ammunition proved to be the best in terms of consistent accuracy, but a few other match ammunition types as well as PMC Bronze, and PMC X-TAC seemed to do quite well in the M400. Below is one of our best groups that we caught on camera using some of my 55-grain Nosler ballistic tipped hand loads.

TFB Review: The Full-Featured SIG M400 SDI XSERIES

I was really hoping that the cheap bulk Winchester White Box 5.56 ammunition I had purchased for part of the review would also prove to be quite accurate out of the M400, however, no such luck. During my accuracy testing with the Winchester ammunition, the best groups I could manage were about 4 MOA in size. During a couple of grouping attempts with several different types of ammunition, the M400 would often start off with one or two shots that were virtually on top of now another, but the remaining 3 shots would string all over the place – even with match ammunition. Admittedly, this started happening only after several groups had been shot and even at their widest, the groups were only about 2.5 MOA wide.

To wrap up the accuracy bit, the SIG Sauer M400 SDI XSERIES rifle is certainly capable of great accuracy, however, it seems to be a bit temperamental as to when it will reach its maximum accuracy potential. Our assumption is that the rifle’s groups are simply opening up as the heat builds up in the rifle’s components, the M400 SDI XSERIES makes use of a slimmer contoured barrel (similar to a Hanson profile barrel) which might be adversely affected in the accuracy department once the rifle heats up a little. All-in-all, I think the M400 SDI XSERIES is plenty accurate. I don’t ever expect AR-15 rifles chambered in 5.56/.223 to be sub-MOA guns or even 1-MOA guns. However, I’d give this rifle more than a passing grade in the accuracy department, given its price range.

Final Thoughts

The SIG M400 SDI XSERIES rifle is certainly aggressively priced for its feature set. The rifle combines great accuracy potential, a unique ambidextrous lower that I’ve come to absolutely love, and a solid reliable platform that you can build into virtually anything you want. In fact, I’d almost go as far as to say that the biggest selling point of the M400 SDI XSERIES is the lower receiver. The SIG MCX SPEAR is the talk of the town these days partially due to its ambidextrous design and I think it was smart of SIG Sauer to bring some of the best features of the SPEAR over to the new SDI XSERIES.

In terms of value, I don’t think it gets much better than this. The $1,300 price tag puts it in line with what most people expect to pay for a “good” AR-15 rifle and the M400 meets all my expectations for reliability, accuracy, and build quality, and even goes a bit further and adds neat and helpful ambidextrous features.

TFB Review: The Full-Featured SIG M400 SDI XSERIES

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