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Tread Heavily: SIG Sauer Steps Up With Its Entry-Level AR

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Another day, another threat to the Second Amendment. Welcome to the post-Sandy Hook political cycle where left-leaning candidates for office play a perpetual game of gun control oneupmanship. America’s favorite rifle, the AR-15, is back in the spotlight as a million-rounds-per-minute ghost gun that gives ER doctors PTSD. While we don’t give points in our reviews for marketing prowess, we have to point out that SIG Sauer’s naming its lowest-priced AR “Tread.” A reference to the Gadsden flag is a fine way to embrace the idea that the AR is both a symbol of freedom and a tool of self-defense.

The M400 Tread is SIG’s entry-level AR, and while it’s not the cheapest AR-pattern rifle on the market, its design, materials, features, and quality of its components set a value versus performance baseline for what an AR should be as we approach the platform’s 60th birthday.

Stock parts (lower left) are easily upgraded with the parts shown: Tread 15-inch handguard, Tango6T optic, Tread Romeo5 red-dot sight, Tread ambi charging handle, Tread 3-chamber brake, and Tread flip-up sights


You can build your own, but unless you’re an accomplished armorer, you’re bound to spend as much as, if not more than, the cost of the Tread by the time you figure out what AR components play nicely together. And, after weeks of workbench time, your build probably won’t have all the features of the Tread. And, most importantly for a novice AR owner, it won’t be warranted to run.

Sig’s Tread-branded flip-up irons are all-metal, lock in the up position, and include the sight-height M-LOK rail section to mount the front sight at the proper height to match the rear iron sight.

Don’t get us wrong — there’s absolutely nothing wrong with building an AR from components. There’s no better way to learn about the ins and outs of the platform. But, at risk of being beaten to death with an armorer’s wrench in the aisles of SHOT Show, we recommend a build project as a second rifle. Getting an out-of-the-box shooter as a first rifle gives you a chance to decide what you like and what you don’t. With that knowledge in hand, you can either start swapping parts on a known, working rifle … or start a component build. Plus, using a running gun as a baseline means you’ve got a lot less to troubleshoot when that new trigger doubles or fails to ignite a primer.

Or you could start out with something like the Tread and take advantage of the company’s modular approach to upgrades and accessories. Instead of leaving its freshly minted AR owners alone in the jungle of unfamiliar specifications and abbreviations, the company released a line of made-for-Tread, factory upgrades that are guaranteed to work.

Choosing compatible parts for the Tread is as simple as finding the Gadsden yellow logo on the box of a SIG accessory. SIG’s EVP of Commercial Sales and Chief Marketing Officer Tom Taylor said, “It’s like Garanimals for guns.”

Instead of getting the runaround of a trigger manufacturer blaming a failure to fire on an out of spec lower receiver, SIG built the whole damned gun, knows all its parts inside and out, and will help you get it running or fix it themselves if you can’t.

The edges of alignment tabs on all three Tread handguards are sharp enough to warrant deburring.

Before we give the wrong impression, though, we’ve run a couple of Tread rifles through a couple thousand rounds over the course of a few months without a single stoppage or mechanical failure.

Bringing this back around to value, we recognize the Tread costs more than rifles at the distal end of the performance and reliability scale, but its price is on par with its competitors in the AR sweet spot, the so-called “Taint” price point.

We’re calling it the Taint space because $700 to $800 rifles aren’t dirt-cheap, nor are they comparable to $1,300 to $1,800 feature-laden AR rifles. Or maybe we’re indulging our inner 13-year-old and embracing the unintended combination of SIG’s Tread branding with that of the competing Springfield Armory Saint line of rifles that occupies a similar market space. Either way, SIG provides a lot of value in the Tread for its $800 street price.

Tread’s out-of-the-box ambi controls include a left-side mag release and an extended, angled bolt release.

Running the gun in its out-of-the-box configuration for several 200-round training sessions over a few weeks set a solid baseline for comparison to the more tricked-out Tread we’d end up with after accessorizing it with SIG’s Tread-labeled upgrade parts.

That polymer wedge takes up an play between the upper and lower.


The first thing we noticed was the slim handguard. The stock M-LOK handguard gets points for ergonomics. Average-sized hands will feel at home with a C-clamp, thumb-over grip or a conventional support-hand cradle. All three Tread handguards have the same profile and omit the top rail, which adds comfort. However, the missing top rail is a drag for guys who want to run a top-mounted aiming laser, such as an ATPIAL or a clip-on night vision setup. Luckily, SIG offers sight-height, M-LOK rail sections to mount irons, lasers, and anything else that’ll fit on a 2- to 3-inch rail section.

We ran the rifle pretty hard in a positional drill that required dumping 15 rounds in prone, kneeling, then standing positions. Five reps and 80 rounds later, the heat from the chamber passed through the barrel, into the unvented portion of the handguard where it built up and crept forward. It was hot enough to call for shooting gloves, though not hot enough to stop shooting while barehanded. The 15-inch accessory handguard ran noticeably cooler with its greater number of M-LOK slots, allowing heat to escape.

Swapping the handguard is a two-bolt operation and takes two minutes if you’re drunk or blind, or both; otherwise, it takes maybe a minute. The setup is similar to Heckler & Koch’s 416 channeled nut and clamping handguard system. It affords a long mating surface for the nut and handguard, reducing the diving board effect seen with shorter barrel nuts. The Tread is equipped with Sig’s micro gas block that accommodates the slimmest of handguards — though your choices are limited to Sig’s at the moment, since the Tread uses a proprietary barrel nut.

It took a while, but we did find an issue with the Tread that was common across all three handguards (stock, 13-, and 15-inch models). They didn’t flex, shift, or fail — they drew blood. At some point while working rifle-to-pistol transitions, the handguard’s rear edge (or the ears that wrap around the upper receiver) caught and left a jagged scratch on our forearm. These areas aren’t dehorned on any of the Tread handguards, and they should be.


The stock trigger surfaces are all DLC-coated for improved wear and feel, but there’s no hiding the heavy, 7-pound, 11-ounce average pull. The best way to describe the break is like popping industrial grade bubble wrap. We’re talking about the kind with the 3/4-inch bubbles. Shooting groups with it demanded a lot of concentration, particularly because we found a tiny hitch in the take-up when shooting very slowly. We were staging the single-stage trigger about a tenth-of-a-millimeter into the pull to find the actual wall before the break. Shooting at speed, it’s unnoticeable. It did make us appreciate Sig’s optional, single-stage Tread upgrade trigger.

Tread’s single-stage upgrade trigger features a flat-ish trigger shoe and a 3-pound, 9-ounce break.

Swapping triggers was as easy as swapping any other non-cartridge trigger — perhaps a little easier since the trigger included a slave pin to hold the disconnector in the trigger body while inserting the trigger pin. It also came with a coned hammer installation pin, making it easier to center the hammer pin before driving it all the way through the receiver. The flat faced, single stage trigger registered a very crisp 3-pound, 9-ounce break. The take-up is long, but absolutely clean with a crisp break rivaling any aftermarket trigger out there. Anyone buying a Tread would do well to consider this as the rifle’s first upgrade.

The ambidextrous, 90-degree safety has a solid, if slightly heavy, feel, and is shorter on the right side to give the trigger finger room to work. It’s easily knuckled backward to safe. The ambi mag release has good leverage when run from the left side, and the angled bolt release prevents its inadvertent activation during vigorous paddle-slapping reload maneuvers.


The carrier key, yawn. Yes, it’s properly staked, as is the castle nut out back. We expect that from Sig.

But what we weren’t expecting was the extractor support in the barrel extension. We consider this a high-end touch and one that Sig uses in its 516 series rifles. When the bolt of an AR is in battery, the extractor is an unsupported area of the bolt head. In an overpressure event, the hinged extractor acts like a relief port and lets gases blow it open. Sig places a pin in the barrel extension that essentially braces the outside edge of the extractor, preventing it from opening when the bolt is in battery. It’s a great safety feature providing insurance against a receiver blow-out if you suffer an obstructed bore malfunction.

Sig Tread’s proprietary barrel nut.

In the lower, Sig includes an Accu-Wedge (or something that looks and works identically) to take up any slop between the upper and lower receiver. We also noticed the Tread lower is the same forged lower used in all of Sig’s AR pattern rifles. It sports thick walls and QD sling sockets on either side at the base of the receiver extension tang. In use, the sockets place the sling’s ring ends just far enough from the hand to work well in most positions, but running the rifle when slung this way doesn’t feel natural if you like to shoot with tension on the sling, as we do.


Odd one, but the manual is pretty helpful for novice AR owners. It calls out all the rifle’s components and explains the cycle of operation, showing how to take the gun all the way down to the extractor spring with very clear diagrams. Anyone with a box of take-off A2 grips and birdcage flash hiders would scoff at this, but we all started somewhere. If the amount of effort Sig put into making the M400 owner’s manual is any reflection of how much effort went into the gun, then we expect few cut corners.


The 16-inch barrel is made of 416R stainless steel and runs a mid-length gas system with 1:8 rifle twist. The recoil impulse is manageable, and the gun is easy to control, even easier with Sig’s three port brake upgrade.

The tread’s 1:8 twist barrel includes a tapered shoulder that reduces concentricity issues when used with tapered muzzle devices.

We’ve put about 2,000 rounds through a couple of Tread rifles in a variety of configurations from bone stock to gussied-up with all the Sig-branded upgrades available, including the Sig Optics Romeo5 red-dot sight and the Tango6T variable power optic. We ran 100- to 200-yard drills with both, shot our accuracy test groups, and moved back to 400, then 500 yards with the Tango6 before we ran out of range. The rifle performed reliably and admirably. We suffered zero malfunctions with this platform.

The stock Tread configuration upgraded with Sig’s Tread Flip-Up Irons Sights and Romeo5 red-dot optic.

Accuracy is on par with any AR in the Tread’s price range, and many above it. Predictably, the faster barrel twist rate favored Federal’s long 77-grain Sierra Match King loads, but it did surprisingly well grouping Black Hills light 52-grain loads. Oddly, though, the Tread basically vomited Hornady’s 73-grain ELD-Match rounds, which are generally good performers.


With a million choices, it’s tough to get excited about another AR. But Sig’s M400 Tread bridges the gap between performance and value, while adding important features for AR snobs like the extension support pin. Whether you’re a card-carrying member of the No Step on Snek crowd or just looking for one of the best values in AR-pattern rifles, the Tread should be at the top of your list.


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