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SIG Sauer M400 TREAD Rifle Review

Introduced in late 2018, the SIG M400 TREAD is still one of the two best AR-­15s on the market priced under $1,000.

SIG Sauer M400 TREAD Rifle Review
Designed to give you everything you want and need in an AR at a reasonable price, with an unusual name and a cool logo, the TREAD is the latest market entry from SIG.

The path SIG Sauer has taken with their AR offerings to me is very reminiscent of Ruger’s journey. Both companies started out making expensive, heavy AR-­15s with all sorts of extra features, but it wasn’t until they began selling a stripped-­down lighter “M4-­style” version that they saw huge jumps in sales. With Ruger, that came with their AR-­556.


SIG was selling their M4-­style M400 rifle quite a while ago, but it was their feature-­laden but affordably priced M400 TREAD, introduced in late 2018, which breathed new life into the line and got a lot of attention — in addition to selling a lot of guns. SIG has since expanded the TREAD line with a pistol and a .308 version, but let’s take a look at the original rifle, which I continue to believe is one of the two best AR-­15s on the market priced under $1,000.


SIG sells several AR-­pattern rifles. Their flagship is the MCX Virtus, which is a piston-­driven gun that has all sorts of bells and whistles. It is also very expensive and rather heavy. At the other end of the spectrum is SIG’s M400 direct gas impingement AR. These rifles have offered a few upgrades over standard Mil-­Spec, but have been priced somewhat higher than similar competing designs from other manufacturers, especially basic “economy” M4-­style rifles.

SIG looked at what they were offering, compared it to the competition, and came up with the M400 TREAD, meant to offer far better than G.I. “M4” looks and performance at a very competitive price. The MSRP is just $951, which means the TREAD will be on dealer shelves probably near or below $850.

The TREAD features many ambidextrous controls. Here you can see the ambi-­mag release and bolt release.

First, let’s deal with the name. TREAD is a bit unusual, but less weird a name for an AR to me than the Springfield SAINT. If you’re wondering where the name comes from look at the logo on the side of the aluminum handguard which is being used all over the marketing materials. The logo is a snake in the shape of the American flag, deliberately reminiscent of the “DON’T TREAD ON ME” Gadsden flag (1775) from the American Revolution. FYI, it was Benjamin Franklin who first used a snake as a symbol of the colonies in his “JOIN, or DIE” cartoon from 1754, so a snake logo has some serious freedom bona fides in this country. Many of the shirts and stickers SIG is selling have the TREAD logo in yellow or on a yellow background, deliberately mimicking the yellow field of the Gadsden flag.

The right-­side safety lever is shorter, so it won’t poke your trigger finger. At the rear of the receiver on both sides you’ll see a QD sling swivel socket, which is a nice extra.

Honestly, I’m surprised SIG doesn’t put the cool TREAD logo on the lower receiver instead of or in addition to the handguard. I know this would cost a tiny bit extra, and this rifle is aimed at a specific price point, but that logo is very cool, and would look great etched into a lower. I’m not sayin’, I’m just sayin’ … .

The first thing about the TREAD that will catch your eye is the stainless steel barrel. Not only is the slender 16-­inch barrel stainless, it sports a 5.56 NATO chamber, 1:8 twist rifling, pinned gas block, and a mid-­length gas system. If you know, you know. If you don’t, let me put it this way … if you were going to spec out an AR-­15 barrel/gas system that you wanted to run reliably under adverse conditions, that could eat whatever ammo you fed it, and was as soft-­shooting as possible, it would be exactly what you get with the TREAD.

SIG’s TREAD logo is a snake in the shape of the American flag. Tarr thinks it is very cool, but believes SIG should have etched it into the lower receiver as well.

The number of ARs on the market priced under $1,000 with a stainless steel barrel is very, very, very low. Provided everything else (metallurgy, machining, etc.) is up to snuff, a stainless steel barrel is superior to other steels.

A 5.56 NATO chamber is a little bigger than the .223 Remington chamber, which in addition to safely allowing you to shoot the higher pressure 5.56 round has a little more room and is thought to provide a little extra reliability when the rifle gets dirty. The 1/8" twist seems to be the best to handle a wide range of bullet weights. And, finally, the mid-­length gas system is considered to be optimum for reliability and reduced recoil in a 16-­inch barrel. Pressures are lower than in the shorter carbine-­length gas system and as a result the bolt moves slower, increasing bolt life and reducing felt recoil.

The medium-­light contour stainless steel barrel has a 5.56 NATO chamber, mid-­length gas system, 1/8" twist, and is tipped by a three-­prong flash hider from SIG.

The barrel profile is generally what determines how heavy or light your AR is going to be. The barrel of the TREAD has a medium-­light profile, and as a result the overall unloaded weight of the TREAD is six pounds nine ounces, even with the long (15-­inch) aluminum handguard. SIG currently lists the weight of the TREAD on their website as seven pounds, but even with an empty magazine in place my sample only weighed six pounds thirteen ounces, so I’m guessing somebody used their Common Core math skills. With the stock fully extended the rifle balances over the front receiver pin, so it is not muzzle heavy. SIG’s ARs have historically been overweight and overpriced, with bulky handguards. The TREAD has none of that.

The barrel is tipped with a three-­prong flash hider of SIG’s own design that seemed to be very effective. The barrel is free floated inside the long handguard. Before we talk about the handguard, let’s talk about what’s underneath it — SIG’s Micro Gas Block. The stainless steel gas block on the TREAD is so fancy at first it looks like it might house an adjustment for the gas system, but the gas system isn’t adjustable.

The QD sling swivel sockets on the receiver are nice, but of more use would have been a better-­than-­GI charging handle. As seen here Tarr installed a SIG TREAD-­branded ambi oversize charging handle and flip-­up sight.

When I was younger I heard Larry Vickers (SFOD-­D Delta Force veteran) say that any AR meant for defensive use should have a pinned gas block — as in the gas block is secured in place by a horizontal pin which actually goes through the barrel. At the time I pooh-­poohed that idea, as I’ve shot a lot of rounds through great guns with gas blocks simply screwed in place, but as I have gotten older I see the wisdom in that statement. If an AR is going to have reliability issues, there’s a huge chance it’s because of a non-­pinned gas block coming loose. While the handguard makes it tough to spot, the Micro Gas Block on the TREAD is in fact pinned in place, so whether you’re just punching paper at the range or the Boogaloo kicks off the TREAD will handle it.


The TREAD features ambidextrous controls and QD sling swivel sockets on either side of the lower receiver at the rear. The pictogram selector markings are nice. Seen here is the standard GI-­pattern charging handle.

The narrow (1.75" x 1.5") fifteen-­inch handguard is a new SIG design, proprietary to the TREAD. It has M-­LOK attachment slots at 3, 6, 9 and 12 o-­clock on the forward two-­thirds of the handguard. If you want to mount a front sight on the handguard you’ll have to first attach a rail section to the top of it near the muzzle, but SIG sells both rail sections and flip-­up sights. In fact, SIG sells all sorts of TREAD-­branded accessories, but let me circle back around to those.

The TREAD wears a SIG-­made pistol grip, oversize polymer trigger guard, and a Magpul SL-­K stock on a six-­position receiver extension that featured a castle nut properly staked in place.

The lower receiver is where you’ll see some serious changes from the GI specs, but done in a very smart, unobtrusive way. The TREAD features what everyone calls ambidextrous controls, and I’ll use that term, but the more correct term is “bilateral” — ambidextrous means you can use both sides equally, bilateral means something has two sides.

First, the TREAD has an ambi safety, with the right-­side lever being short enough that it didn’t poke my trigger finger, a problem I have with most ambi-­safety levers. The rifle also has an ambi-­magazine release, with a pivoting lever on the left side, and an extended bolt release to make locking the bolt back a little bit easier.

To test its reliability Tarr used a wide variety of magazines. The magazine well on the TREAD was rather tight, and the Gen 1 Magpul PMag didn’t drop free, but reliability with all magazines and ammunition was 100%.

Now we get to one of my few complaints about the gun: just ten percent of the population is left-­handed, but everyone who shoots an AR has to use the charging handle, so I don’t know why SIG would equip the TREAD with improved ambi controls and then stick a single-­sided and outdated GI-­style charging handle in the gun. A flared magazine well would have been more useful to everyone as well. I mention that not only because I think a slightly flared magazine well is needed on the AR in general, but because the magazine well on the TREAD was noticeably tight. If this was my gun I would take a Dremel tool to the mag well opening just to increase the bevel there a bit.

There are big QD sling swivel sockets on either side of the lower receiver at the rear. There is a rubber tensioning device at the back of the lower receiver underneath the rear receiver pin to eliminate any play or rattle between the two receiver halves.

The caliber and barrel twist rate is marked on the barrel near the muzzle. Here Tarr has installed the alternate 15-­inch SIG TREAD-­branded handguard and flip-­up sight on the rifle.

According to SIG, the TREAD is equipped with a “single stage polished/hard-­coat trigger” and I wish I could say it provided a trigger pull as good as that sounds, but no. Trigger pull on my sample was seven pounds even, and a tiny bit gritty. A seven-­pound trigger pull is great if you’re using a heavy trigger pull as additional insurance against a negligent discharge, but it sucks if you know what you’re doing or, you know, actually want to shoot tight groups at distance.

Think I’m a spoiled gunwriter and a trigger snob? Well, you’re not wrong, but it’s people who don’t know any better who think a GI-­style gritty six-­ or seven-­pound trigger pull on an AR is A-­OK. Those people who do know better replace those triggers. Example A: Geissele Automatics. Those “people who know better” (cough*Delta Force*cough) went to Bill Geissele and asked — begged — him to design a better trigger for their M4s, and thus the SSF trigger (with a crisp 4.5-­pound pull) was born.

Using some of the many SIG TREAD-­branded accessories, Tarr upgraded his rifle. He installed the more skeletonized 15-­inch M-­LOK handguard, handstop, flip-­up sights, and ambi oversize charging handle.

With all of that whining out of the way, I will say that putting in a better trigger would raise the price of the TREAD, probably significantly, and this is, among other things, a price point gun. I’m guessing SIG did everything they could to improve this rifle, while aiming for an MSRP under $1,000. Luckily, the number of improved aftermarket triggers for the AR-­15 platform is in the hundreds.

The provided stock on the TREAD is Magpul’s newer SL-­K, which has a reduced profile and a rubber butt pad. It rides on a six-­position Mil-­Spec buffer tube. I was pleased to see the castle nut, which holds the buffer tube firmly to the receiver, was properly staked in place.

The pistol grip is one of SIG’s own design that mimics the A2 grip angle while providing a little more material under the web of your hand to promote proper placement of your finger on the trigger. The trigger guard is an oversize polymer piece. The rifle is supplied with one 30-­round magazine, a Magpul Gen 2 PMag in the case of my sample.

Swapping out one SIG handguard for another requires only removing two hex-­head bolts which secure the handguard to this barrel nut. The barrel’s light profile is apparent.

The AR-­15 is often described as “LEGOs for men” as it is possible to swap out every part on the gun — EVERY part — using just simple hand tools. This is, in fact, one of the great things about the design, its modularity. It’s tough to find two people whose ARs are identical, as they are so easy to customize. Why does this matter when we’re talking about a “factory” gun?

SIG seems to be pushing their “TREAD-­branded” accessories and upgrades just as hard as they are the rifle. Most of the photos of TREADs I see on SIG’s website show the rifle sporting some of those accessories, such as triggers, muzzle devices, flip-­up sights, ambi charging handle, vertical grips/handstops, and the aforementioned alternate aluminum handguards.

Under normal circumstances I would say that swapping out parts and accessories on an AR is the same from gun to gun, but in fact SIG has made things a little easier if you want to replace the provided handguard on the TREAD with one of their two alternate models.

SIG’s gas block is so fancy at first you might think it is adjustable, but it is not. It is pinned in place for the utmost in durability and reliability. That blowback and discoloration on the gas tube is normal after you’ve put hundreds of rounds through a gun.

Every SIG TREAD handguard uses the same barrel nut, and all you have to do to swap handguard is to remove two hex-­head bolts securing the handguard to the barrel nut. Personally, I think the alternate 15-­inch handguard with lightening cuts SIG sells as a TREAD factory accessory is the one they should have put on this rifle, as it is both more functional (more M-­LOK slots) and has a more traditional appearance than the skinny factory standard handguard.

While I don’t really care for the looks of the provided handguard it works just fine, and even as a whiny spoiled gunwriter the only minor complaint I have with the base rifle is the unimpressive trigger and old-­school charging handle. Well, one more — I wish it came with iron sights, but yes, I know, those would cost additional money. Subscribing to the “everything that can go wrong will” philosophy, I believe every rifle intended for self-­defense use should have backup irons no matter how reliable your optic of choice is.

For accuracy testing, I mounted a Trijicon Accupower 1-­8X scope in a quick detach Midwest Industries mount, and the rifle did very well. Accuracy with just about every type of ammo I tried averaged between 1-­2 MOA, and the biggest challenge was shooting up to the rifle’s accuracy potential, because of the seven-­pound trigger.

A closer view of some of the SIG TREAD-­branded accessories installed on the gun — the 15-­inch skeletonized M-­LOK handguard, the handstop, and the flip-­up front sight. This handguard allows you to see the narrow barrel profile, which helps keep weight down.

For up close speed work I mounted the SIG ROMEO4T optic on the gun, which I have previously reviewed in these pages. This small red dot is built to handle military grade abuse, and only adds a few ounces to the weight of the gun, unlike the Trijicon 1-­8X scope and mount combo which adds 34 ounces. Although … that weight helps to eliminate what little recoil there is, and that 8X magnification will allow you to use the rifle out to the limits of the cartridge’s capability.

I grabbed an assortment of magazines from my basement for reliability testing. In addition to the provided Gen 2 Magpul PMag I used a Gen 1 PMag, a Lancer AWM, and a Colt aluminum magazine with a Magpul follower. I specifically grabbed a first-­generation Magpul PMag as they are a little larger than the new ones, and as I mentioned the magazine well of the TREAD was tight. The Gen 1 PMag fit in the gun, but it did not drop free when empty no matter whether the bolt was closed or locked open.

Accuracy testing is necessary, but not exactly fun. Fun for me is practicing snapping the rifle up to my shoulder and seeing how fast I can hammer silhouettes at various urban distances, practicing my target transitions (the lack of forward weight on this rifle made it easy to swing back and forth and stop the muzzle where I wanted to), and shooting off various improvised rests — 55-­gallon plastic drums, walls, a knee, etc.

SIG sells a number of TREAD-­branded accessories for the rifle. Seen here are just a few — 13-­ and 15-­inch skeletonized hanguards, sights, a match trigger, ambi charging handle, muzzle brake, and more.

For the bulk of my range fun I used bulk-­packed 5.56 NATO FMJ from Frontier Ammunition. A collaborative business partnership between Hornady and the Lake City ammunition plant (normally used to load government rifle ammo), Frontier ammo features Hornady bullets loaded into cases by the Lake City plant for budget-­priced ammo.

I would like to point out that, for this article, I was getting myself reacquainted with this rifle. This rifle was sent to me quite some time ago, and was in fact used throughout the entire 2019 season of the Handguns & Defensive Weapons TV show on The Sportsman Channel, and we’ve been using it while filming the 2020 season as well. I and my co-­host Rich Nance have put untold hundreds of rounds through this rifle at the legendary Gunsite (which is in the dusty high desert of Arizona), The Site training facility in northwest Illinois, and the private range run by Pendleton Safes in Georgia. I’m shocked it doesn’t look worse than it does, and in fact don’t know how I’ve managed to keep it from getting scratched up.

In all that time, the rifle has never jammed, never malfunctioned in any way. And I have yet to clean or even lube it.
Long ago SIG sent me a box of their TREAD-­branded accessories, and for this article I opted to install the ones I liked to fully customize the rifle, as I’ve decided that, whenever SIG gets around to asking for it back, I’m going to buy it (which should tell you how I feel about its value).

The TREAD upgraded with the SIG TREAD-­branded accessories. The accessories Tarr chose make the TREAD “perfect” in his opinion — they provide added performance and improved looks, but combined have an MSRP of $376, which is a significant bump.

Exactly what accessories does SIG offer for the TREAD? 15-­ and 13-­inch skeletonized M-­LOK handguards, an all-­metal flip-­up sight set, ambidextrous oversize charging handle, two-­stage match trigger with a flattish face, handstop kit, three-­port muzzle brake, and vertical foregrip kit, at a minimum. I chose to install the 15-­inch handguard, flip-­up sights, handstop, and the ambi charging handle. You can see the result in the photos.

The accessories I chose I feel make the TREAD “perfect” — they provide added performance and (in my opinion) improved looks, but combined have an MSRP of $376, which is a significant bump.

No honest review of the SIG TREAD would be complete without discussing the one rifle that I believe is the direct reason for the existence of the TREAD and several other new ARs, the Springfield Armory Saint. When it was introduced three years ago the Saint offered an AR-­pattern rifle that was significantly improved over a standard “M4-­pattern” AR, sporting better than GI furniture and a mid-­length gas system instead of the traditional carbine-­length. Springfield wasn’t the first company to do this, but the Saint was close to three hundred dollars less than any competing rifle from any other major manufacturer. And it had a huge impact on the market.

Ruger responded with their AR556 MPR, and SIG followed suit with the TREAD. It’s said that a rising tide lifts all boats, and with companies struggling to reignite interest in the stagnant AR segment even with the coronavirus-­related gun sales surge (most guns sold were handguns suited for CCW), the American consumer is the real winner.

Personally, I want every American to own an AR, so that when they come for one they have to come for us all, and there are few (if any) better choices for the money on the market today than the SIG TREAD.

James Tarr is a longtime contributor to Firearms News and other firearms publications. He is also the author of several books, including CARNIVORE, which was featured on The O’Reilly Factor. His brand-­new novel, Dogsoldiers, is available now through Amazon and Barnes & Noble.

SIG TREAD M400 Specs

Caliber: 5.56 NATO
Weight: 6 lbs. 9 oz
Overall length: 32.5 in. (stock collapsed); 35.75 in. (stock fully extended)
Receiver: Forged aluminum
Barrel: 16" stainless steel, mid length gas system, 1/8" twist
Muzzle Device: SIG 3-­prong flash hider
Stock: Magpul SL-­K
Pistol Grip: SIG
Forend: SIG 15-­inch M-­LOK
Trigger: 7.0 pounds (as tested)
Sights: None
Accessories: One 30-­round magazine
MSRP: $951.00
Manufacturer: SIG Sauer;

SIG TREAD M400 Rifle Accuracy Results

Accuracy results are the averages of four five-­shot groups at 100 yards from a sandbag rest. Velocities are averages of ten shots measured with an Oehler Model 35P 12 feet from the muzzle.

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