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A brand-new SIG M400 TREAD with a ROMEO5 optic
Out of the box and into a cold, cold world. A brand-new SIG M400 TREAD with a ROMEO5 optic before some winter tweaking and testing. Michael R. Shea

SIG is a company that likes to stand out.

In the crowded field of sub-$1,000 AR-15s, they’ve done that by packing features into the M400 Tread that usually come on rifles costing twice as much. Just how crowded is the AR marketplace? Take the online gun store Cheaper Than Dirt.

They have 398 AR models for sale currently. Filter that long list by 5.56 NATO and .223 Rem. caliber long guns, and you still have 239 options—ranging from a basic Radical Firearms for $440 to a Colt Commando Retro Carbine for $2,366. In the $500 to $1,000 bracket—where most shooters shop for their first AR—there are 119 rifles to choose from.

SIG knows this, so they set out to build the very best sub-$1,000 rifle available. After six months of shooting the M400, I think they might have done it.

The SIG M400 Tread in stock configuration
The SIG M400 Tread in stock configuration brings features normally found in AR-15s that cost several hundred of dollars more. Michael R. Shea

In 2010, SIG released their first stab at the traditional direct-impingement AR-15 with the original M400. Both upper and lowers were high-quality 7075-T6 billet, opposed to cast of 6000-series metal. Quick detach points were milled into the lower, along with more traditional sling swivels. The magazine release and selector were both ambidextrous, and the mag well was flared for faster mag changes. It shipped with a very good stainless barrel and retailed $1,400.

The new M400, which costs several hundred dollars less, retains all these features, including the mid-length gas system, rather than carbine-length found on most other ARs at this price. (Mid-length is generally better for a variety of reasons related to pressure, bolt speed, and dwell time, which is the amount of time the BCG is receiving enough gas pressure to cycle the action—it’s determined by how long the bullet is in the barrel after passing the gas port. When it does, gas is syphoned into the action through the gas port. That pressure is released when the bullet exits the muzzle.)

This new M400, released at SHOT Show 2019, is also a part of the TREAD family of branded SIG products like muzzle brakes and handguards, which adds up to something like a starter set for the aspiring home AR builder.

Man aiming Sig M400 TREAD in snow.
Three hundred rounds in, and the M400 TREAD hasn’t missed a beat. Michael R. Shea

“But all AR parts are compatible already!” says the Internet experts.

Correct. Sort of.

You still have to know what you’re doing and you have the right tools. And I reckon there are a lot of people, like me, who took their AR apart without either and had springs shooting here and there, not to mention buying AR-10 parts by mistake or mil-spec instead of standard.

The TREAD lineup more or less eliminates the need of special knowledge or special tools.

For example, any aftermarket handguard can be attached to just about any AR, 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.

Removing the handguard was a matter of pulling four screws.
Removing the handguard was a matter of pulling four screws. Michael R. Shea

Remove the old handguard and barrel nut, then install the new handguard to the new barrel nut. Here’s how its done step-by-step. If you’ve got all the tools and know what you’re doing, it doesn’t take very long. But fair warning: you’re going to cuss a lot. I certainly did on my first one.

With the TREAD, all the available handguards use the same barrel nut, so swapping is just a matter of turning Torx screws in the handguard itself, which all thread directly into the barrel nut—the handguard isn’t just tension fit—so it’s good positive purchase, and won’t walk under fire.

After some plinking with the stock M400, a friend and I set about upgrading it with new parts from SIG, and a few other do-dads to tailor the rifle for winter predator hunting. The parts we used included:

· TREAD 15-inch Skeletonized Handguard

· TREAD Ambidextrous Charging Handle

· TREAD 3-Chamber Compensator

· TREAD Vertical Grip Kit

· Maven RS.1 2.5-15×44 Optic

· Magpul MBUS Pro Offset Front and Rear Sights

· Timney AR-15 Competition Trigger

Installing the TREAD 15-inch skeletonized handguard.
(top) The same screws that we removed to take off the old handguard are used to install the TREAD 15-inch skeletonized handguard. The screws thread directly into the barrel nut for a secure installation. (bottom) The TREAD 15″ skeletonized handguard fully installed. Michael R. Shea

Next, we swapped out the stock three-prong flash hider with the TREAD 3 Chamber muzzle brake. Again, any AR-threaded 1/2×28 brake will thread on 99 percent of AR barrels. Issues, albeit aesthetic, arise when the taper of the barrel doesn’t match the brake. The SIG brake comes with a tapered crush washer, which matches the barrel contour. It comes with a standard washer, too, if you ever want to use it on straight-end tube.

Installing the TREAD 3-chamber compensator.
Installing the TREAD 3-chamber compensator. Michael R. Shea

In less than an hour, the rifle looked very different with new handguard, comp and grip kit. We took it to the range with three boxes of very inexpensive ammo and some match Berger’s from Federal. Despite the cold 10 mph wind, it shot well for an AR. The American Eagle 50-grain JHP have become my go-to coyote load.

Taking the Tread for a test shoot topped with a Maven RS.1 2.5-15x44 optic.
Taking the Tread for a test shoot topped with a Maven RS.1 2.5-15×44 optic. Michael R. Shea

SIG prides themselves on disrupting the firearms market. With the M400 TREAD they’ve done that, quietly offer the best value available in the $500 to $1,000 class. It snaps together tight, shoots well, and feels good. Highly recommended as any new shooters first AR, or the aspiring home builder.

For the Complete Range365 Guide to Building an AR Rifle, go here!


SIG Sauer M400 Tread SPECS:

  • OPERATING SYSTEM: Direct Impingement
  • CALIBER: 5.56 NATO
  • BARREL LENGTH: 16 in (406 mm)
  • BARREL MATERIAL: Stainless Steel
  • TWIST RATE: 1:8
  • LANDS & GROOVES: 6
  • MUZZLE THREAD: 1/2 in – 28 UNEF
  • RECEIVER FINISH: Hard Coat Anodized
  • ACCESSORY RAIL: M-LOK
  • STOCK TYPE: Magpul 6-Position SL-K
  • TRIGGER TYPE: Single Stage Polished/Hard-Coat
  • TRIGGER PULL WEIGHT: 5.5 to 9.5 lbs.
  • MAGAZINE (SHIPPED): Magpul PMAG 30
  • OVERALL LENGTH: 30.8”
  • OVERALL WIDTH: 2.5”
  • HEIGHT: 7.5”
  • WEIGHT: 7 lbs.
  • MSRP: $949
  • REAL WORLD: $799

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