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SIG M400 TREAD AR Pistol Review

SIG Sauer's newest AR pistol – the SIG M400 TREAD – is short, sweet and ready to serve today's law enforcement officers.

SIG M400 TREAD AR Pistol Review
Photo by Mark Fingar

Last year, SIG Sauer introduced the TREAD version of their M400 AR-15, and anyone paying attention to industry trends could have predicted their next move. SIG has now introduced a pistol version of the TREAD.

In a flat sales market, the one segment of ARs that continues to do well is AR pistols. Their increased utility pertains to the law enforcement (LE) side as well. Before we dive into the reasons why your department should look at AR pistols instead of rifles, let’s look at the specs of the TREAD pistol.

SIG sells all sorts of ARs. Their most expensive model is the MCX Virtus, which is a piston-driven gun that has all sorts of bells and whistles. It is also expensive and rather heavy. They also have their M400 direct gas-impingement AR. These rifles offer a few upgrades over standard Mil-Spec rifles but have been priced somewhat higher than basic “economy” AR-15s.

SIG-M400-1
Photo by Mark Fingar. To make the SIG TREAD as accurate as posible, SIG gave it a polished, hard-coated, single-stage trigger.

What’s A TREAD?

Enter the SIG M400 TREAD, which turned out to be a very successful addition to SIG’s AR line. It’s meant to be a value-priced AR that gives you everything you need and a little bit more. If you’re wondering where the name comes from, look at the logo in their 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 (circa 1775) from the American Revolution.


The SIG M400 TREAD pistol is a direct-impingement-­operated AR-15 with an 11.5-inch barrel and a carbine-length gas system. While the rifle has a stainless steel barrel left in the white, the pistol has a carbon steel barrel with a black-nitride finish. The barrel has a 5.56 NATO chamber, 1:7-inch twist and is tipped with an A2-style flash hider.


The 10.25-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. Under the handguard is a low-profile gas block made by SIG. The gas block is of an unusual and somewhat fancy design. It is pinned in place for the utmost in reliability.

SIG-M400-2
Photo by Mark Fingar. The TREAD pistol has bilateral controls that are sized larger for better operation by both right- and left-handed officers.

SIG has equipped the TREAD rifle and pistol with what everyone incorrectly calls ambidextrous controls. The proper term is “bilateral.” The TREAD has a bilateral safety selector, with the right-side lever being short enough that it won’t poke your trigger finger, a problem most righties have with right-side safety levers. The TREAD also has a bilateral 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.

There are quick-detach sling-swivel sockets on each 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 pistol grip is a SIG 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 triggerguard is an oversize polymer piece. The rifle is supplied with one 30-round Magpul Gen 2 PMAG. The TREAD pistol is advertised as weighing 6 pounds with the empty PMAG in place.


According to SIG, the AR pistol is equipped with a “single-stage polished/hard-coat trigger.” These polished or nickel-boron Mil-Spec-type trigger groups provide a better than average GI-style trigger pull, all while being safe and durable and fit for duty. Trigger pull on my sample was a crisp, grit-free 6.5 pounds.

SIG-M400-3
Photo by Mark Fingar. The Shockwave 2.0 brace has six easily adjustable settings.

At the rear is what makes this pistol a pistol, the Shockwave Technologies’ Blade 2.0 arm brace. This is a vertical piece of polymer that adjusts for length on the six-position receiver extension. At full extension, the back of the brace is 13 inches from the face of the trigger. You are supposed to press it against the inside of your forearm to provide additional support while shooting.

As a reminder, the Bureau of Alcohol, Tobacco, Firearms and Explosives (BATFE) has made it clear that as long as you do not otherwise modify the brace, shooting a brace-equipped pistol from the shoulder does not alter the legal definition of the firearm. It remains a pistol, which means it is legal whether you’re a citizen or a police officer.


SIG made the TREAD series fully customizable with TREAD-branded sights, handguards, upgraded trigger groups, charging handles and other accessories. My pistol sample sports the upgraded bilateral charging handle and the TREAD-branded SIG ROMEO5 optic.

SIG-M400-5
Photo by Mark Fingar. A SIG ROMEO5 red-dot sight comes with each pistol. The robust sight features a 2-MOA dot and 10 illumination settings.

What Matters Most

I’ve done a lot of testing of short-barreled ARs, and my preferred barrel length on SBRs/AR pistols chambered in .223/5.56 is the 11.5 inches seen on the TREAD pistol. If the barrel is shorter than this, your velocities drop off drastically. Any barrel longer than 11.5 inches ends up not being much shorter than an actual rifle. This length seems to be the best of both worlds. Also, the increased distance between the muzzle and the gas port on an 11.5-inch AR has been shown to make them more reliable than 10- and 10.5-inch ARs.

The accuracy of any rifle isn’t necessarily determined by the length or thickness of the barrel. The short and somewhat slender Government profile of the TREAD pistol isn’t as much of a handicap to shooting accurately as is the 6.5-pound trigger. Still, when I was doing my job at the bench, I could get most ammo to group under 2 inches. For accuracy work, I mounted a Trijicon AccuPower 1-8X scope on the pistol. However, most of my testing (defined as having a lot of fun hammering silhouette targets at urban distances) was done with the budget-­priced TREAD-branded ROMEO5 optic sent to me with the pistol. The pistol ate everything I fed it.

My preferred round for short-barreled ARs is Black Hills’ 5.56 NATO 50-grain Optimized TSX load. This cartridge was specifically developed for a federal agency’s short-­barreled ARs. Because it is light for the caliber, the 50-grain load is fast out of every length barrel, and the Barnes solid-copper projectile holds together while punching through auto glass or car doors. The “Optimized” in the name means Barnes specifically optimized this bullet to perform well when shooting through intermediate barriers. It expands well and penetrates ballistic gel to roughly 14 inches no matter what.

SIG-M400-4
Photo by Mark Fingar. One of the best bilateral additions SIG has on the TREAD is the dual-roll-pin designed charging handle.

Duty Advantages

OK, now why should you or your department be interested in an AR pistol over a rifle, or even a short-barreled rifle (SBR)? Let’s tackle the SBR question first.

Modern AR pistols with their adjustable braces (and the BATFE ruling on shooting from the shoulder) are just as functional as SBRs while not requiring the same kind of federal paperwork as SBRs, which are Class III National Firearm Act items. Many police administrators aren’t interested in guns be it SBRs, select-fire weapons or other firearms that require additional work and paperwork on their part. Pistols require no special hoops to be jumped through. That’s why individual officers can buy them as personal weapons without requiring any departmental letterhead.

As for why you might want a pistol over a carbine — length matters. Whether we’re talking about storage space in the front of a squad car or length of a shouldered firearm while going through doorways, pistols are much more compact. Also, many departments are seeing the wisdom of mounting suppressors on their long guns. Those suppressors add both length and weight, so sticking them on the end of an AR pistol means the end result is just about the length of a standard AR rifle but much easier on the ears. It’s a win-win.

Remember, because it is a pistol, it is illegal to mount a vertical foregrip on the handguard of the TREAD pistol, even if you are a police officer. However, angled foregrips and handstops are perfectly legal.

Many rifle companies are competing to offer the best AR pistols possible, and the feature-rich SIG TREAD pistol is a unique piece and a solid value.

SIG M400 TREAD Specs

  • Type: Direct-impingement, semiautomatic 
  • Cartridge: 5.56 NATO 
  • Capacity: 10, 20, 30 rds. 
  • Barrel: 11.5 in., 1:7-in. twist 
  • Overall Length: 27 in. (collapsed), 30.5 in. (extended) 
  • Weight: 6 lbs. (with empty magazine) 
  • Brace: Shockwave Blade 2.0 
  • Grip: Polymer 
  • Trigger: 6.5 lbs. (tested) 
  • Muzzle Device: A2 flash hider 
  • Sights: None 
  • MSRP: $951 
  • Manufacturer: SIG Sauer, sigsauer.com
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