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SIG SAUER M400 Tread

The SIG Sauer M400 Tread takes the 'economy' AR to the premium level.

SIG SAUER M400 Tread

Photos by Mark Fingar

There has never been a better time to be in the market for an AR-­15. The current political administration is friendly to gun owners, and there has never been a wider selection of well-­priced rifles.

Among all those beautiful AR-­15s exists a demographic of “economy” rifles selling for less than $1,000. Standing out in this crowd is no easy chore, but no AR that sells for less than $1,000 has anywhere near the performance or value of SIG Sauer’s new M400 Tread.

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Usually unheard of on an economy rifle, both the safety and magazine release are ambidextrous.

“Economy” Differences

There are a handful of AR-­15s out there that sell for less than the M400 Tread and more that sell for about the same amount. All fall into the “economy” category. However, the list of features separating the Tread from its competition is long and compelling.


Any of the rifles that cost less than the Tread have a 1:9-­inch-­twist, chrome moly steel barrel. Most of these barrels are manufactured and sold almost as a commodity. Consistency between barrels and accuracy is given very little thought because those things cost money, and that’s not important.


SIG Sauer’s barrel is made from stainless steel, a huge step up in performance from a material selection process alone. Stainless steel is much easier to shape and cut than chrome moly steel because it’s softer and machines easier. This is why stainless barrels are more accurate and consistent, and all custom match barrels are made from this material. Stainless steel is also more expensive.

SIG Sauer managed to fit a stainless steel barrel on this AR rifle and took the time to put the right twist rate on it (1:8 inches) and a midlength gas system. The 1:8-­inch twist rate is a sweet spot for the .223 Remington.

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The Tread’s vertical forward grip is an optional accessory.

Gas System Brilliance

The presence of a midlength gas system on an “economy” AR-­15 is cause to celebrate. The go-­to gas system on just about every low-­cost AR is a carbine-­length system with a fixed front sight post because that’s the cheapest possible arrangement.

A fixed front sight post will block the view through a red-dot sight and prevent easy installation of a free-­float handguard down the road. It also limits handguard length to 7 inches. This means positional shooting will suffer because there isn’t a lot of real estate up front for the shooter’s support hand or for field rests to help stabilize the rifle. Fixed front sight posts and 7-­inch handguards should absolutely be avoided on any AR.


A carbine-­length gas system is also harder on the bolt than a longer system and leads to shorter bolt life. As the bullet moves down the barrel, it passes the gas port, sends gas back down the gas tube and into the receiver where it unlocks the bolt and cycles the action. The closer the gas port is to the chamber, the higher the pressure will be inside the gas system when the bolt unlocks and cycles.

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SIG’s Micro Gas Block comes standard on every rifle.

Pressure inside the gas system causes the bolt to have binding force on it when it twists to unlock. The higher the pressure, the higher the binding force. Binding force causes the bolt lugs to drag across the abutments inside the barrel extension. The more force that’s applied, the shorter the bolt lug life will be. Since the lugs are the first things on an AR bolt that break, some care in avoiding binding force is wise.

SIG Sauer knows all of this, and that’s why they slapped a midlength gas system on the Tread. This is the only AR in this price bracket that offers a midlength system of which I’m aware. The merits of the longer gas system are many and should influence any rifle selection procedure. This reason alone is enough to put the Tread on top of any economy-­rifle comparison.


Give It a Hand

Surrounding that beautiful barrel and gas system is a free-­floating handguard. They’re quite rare on ARs costing less than $1,000. The handguard that ships on the Tread is a simple extrusion that has a shape conducive to good positional shooting.

Most economy-­class ARs have the standard plastic two-­piece circular handguard that snaps between the D-­ring and the housing attached to the front sight post. The shortness of the handguard is a problem because there isn’t much to rest on objects. Resting the rifle is the best way to stabilize it and greatly increases accuracy.

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SIG took the time to do the small stuff that differentiates the Tread from the other “economy” rifles.

The Tread’s handguard is 15 inches long, and in addition to the great length, it’s flat on the bottom. Anything like a fence rail, bumper or even a tree limb allows the flat bottom to assist in keeping the rifle stable when laid across it. Round objects, though supported, like to roll around, which makes accurate shooting harder. It’s such a simple thing to put a flat surface on the bottom of the handguard, but so far SIG Sauer is the only company I’m aware of to do it on an economy rifle.

SIG Sauer offers a couple aftermarket handguard options for the rifle and markets this ease of interchangeability with some emphasis. The internet haters squawk and carry on about how all AR accessories fit together, but there’s a big “but” attached to that. The best example to demonstrate the ease of the Tread system is the handguard.

Any aftermarket handguard can be attached to just about any AR rifle, provided you have an action block/rod, a vise, a barrel nut wrench (for both the old nut and the new one) and a torque wrench.

Remove the old handguard and barrel nut, then install the new handguard to the new barrel nut. If you’ve got all the tools and know what you’re doing, it doesn’t take very long. Everyone else is going to cuss a lot.

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With the addition of a SIG Romeo5 red-dot sight, back-up irons and a vertical forward grip, the Tread is capable for most any task.

Where the Tread differs is that the system allows you to leave the barrel nut in place while still changing handguards to fit your preference. The convenience of not having to remove and replace barrel nuts should not go unnoticed. Doing so is not an overly complicated task, but it does take the right tools and some time. Also, if the barrel nut is torqued too tightly or the right tools aren’t used, the guy doing the parts exchange can bend the upper receiver or strip threads on the tenon. If the gas tube doesn’t go into the upper receiver correctly, erratic accuracy will result.

I like the handguard that ships on the upper receiver enough that I would leave it alone, but two other options from SIG are available. The optional handguards have machined cuts into the sides and bottom that continue the entire 13-­ or 15-­inch length. The additional machine time to make them would bump the cost of the rifle up, that’s why they are optional.

All it takes to swap out handguards is the removal of two screws. Since all of the options share the same barrel nut, the process of changing one out takes about two minutes if you take your time.

Behind the Rifle

One of the first things to notice about the Tread is the ambidextrous controls. Usually unheard of on an economy rifle, both the safety and magazine release are ambidextrous.

Not only does the rifle have an ambi magazine release, it has a lower receiver designed to work with it. The easy thing to do would have been to slap a Norgon ambi drop-­in magazine release (or something similar) on there and call it a day, but SIG Sauer went to the trouble of machining a fence around the ambidextrous magazine release. The fence around the magazine release dates back to the 1960s when the military called for one after completing some early testing on the M16. Magazine releases without a fence have a bad habit of dropping the magazine at the worst possible time.

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There was no disagreement with the Tread’s smooth, single-stage trigger. But if you want to replace it, SIG offers a single-stage trigger replacement kit ($119) that features a flat-blade trigger shoe.

In addition to the sweet controls and a lower receiver designed to support them, the company put an extended bolt release on the left side that sits inside the same fence that surrounds the magazine release. This makes it easier to release the bolt in a hurry while still protecting the bolt catch from activating accidentally. There are two integral, quick-­detach sling-­swivel sockets machined into the sides of the lower receiver (one on each side) for those who favor single-­point slings.

Accuracy was better than your average AR with groups in the .63-­inch range. Considering the average five-­shot group size for a chrome-­lined Mil-­Spec barrel sits right around 1.2 inches, that’s excellent for an economy AR. Such is the effect of a well-­made stainless steel barrel.

While the M400 Tread is SIG’s latest foray into the price-­friendly AR zone, the list of features screams “premium” in any language. In addition to the excellent components selection, build quality is at that same level. Everything is staked correctly, finishes are applied evenly and all signs indicate this rifle will give years of reliable service. If you desire performance and value, the Tread should be at the top of your shopping list.

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