The SIG Sauer TREAD series of AR-15s is advertised as being “easily customizable, with a full line of purpose-built accessories, designed and built in the USA.” That ability to customize their rifles is important to a lot of AR users, as evidenced by an ever-expanding number of aftermarket parts and accessories. While it is true that the TREAD series is easily customizable, I like how the rifles are set up right out of the box. SIG’s 5.56mm M400 TREAD is just about as ideally set up as it gets. The only things needed in my book are optics/sights, a weapon light and a single-point sling.

About the M400

Customization of the M400 TREAD is easily accomplished largely because of the slim, free-floating M-LOK aluminum handguard. When the Magpul M-LOK forend came out in 2014, bulky Picatinny quad rail forends all but disappeared from the market. Users were able to have a greatly slimmed down, easy-to-grasp forend and attach only small segments of rail, allowing only the accessories that each shooter needed. This eliminated long, unused sections of railing that were uncomfortable to grasp unless covered. The SIG Sauer M-LOK forend is particularly nice due to its contoured angles that narrow toward the top, providing an ideal surface to lay the thumb of the support hand while firing. The bottom of the forend is flat for a more stable hold when firing from a rested position.

SIG does a great job with its ambidextrous operating controls. The safety/selector and magazine release are both ambidextrous, as are the QD sling mounting points at the rear of the receiver. While the bolt stop release isn’t ambidextrous, it is enhanced with an extension for easier manipulation. The Magpul M4 stock adjusts very easily by ambidextrous release levers. SIG Sauer has done nearly everything possible in terms of engineering the M400 TREAD to be as user-friendly for left-handed shooters as it is for right-handed shooters.

M400 Specifications

Caliber: 5.56 NATO
Overall Length:
27 inches
Overall Width:
2.5 inches
7.5 inches
6 pounds

Interestingly, the M400 TREAD’s flash suppressor is a three-prong or “duckbill” type. The three-prong suppressor does very well at what it is designed for, though it was replaced by the “birdcage” type on the later M16A1 model due to its propensity for clogging if the rifle was dropped muzzle-first. This suppressor delivers an audible “ping” sound each time the rifle is fired because of its tuning-fork-like design. It is quite cool, actually.

The M400 at the Range

For testing, I mounted the Crimson Trace CTS-1000 Compact Tactical Red-Dot sight I had previously tested on a Ruger. The CTS-1000 utilizes a quick disconnect rail mount and features a 2.0 MOA Red Dot with 10 levels of brightness. It is an excellent sight for close-quarters battle. I used SIG .223 77-grain OTM ammunition for testing.

At 100 yards, the 2.0 MOA dot covers more area on the silhouette target than I like when trying to get an idea about precision capability. So I tested at 50 yards, resting the M400 TREAD on an adjustable Caldwell lightweight rest. I easily shot six-shot groups in the 2-inch range once zeroed in. And I was definitely aided in firing accurate shots by the polished, hard-coated, single-stage TREAD trigger. Functioning, of course, was flawless.

Wrap Up

The SIG Sauer M400 TREAD is a remarkable value considering it is as full featured as it gets. It is a great companion arm to back up the .308-caliber 716i TREAD rifle I tested previously. Bass Pro Shops has the M400 version available for $799.99.


SIG Sauer:
Crimson Trace:

About Scott W. Wagner

Scott W. Wagner is a criminal justice professor and police academy commander from Columbus, Ohio. He has been a police officer since 1980, working as an undercover liquor investigator, undercover narcotics investigator, patrol officer, SWAT team member, sniper and assistant team leader. Scott is currently a patrol sergeant with the Village of Baltimore, Ohio, Police Department. He has been a police firearms instructor since 1986 and is certified to instruct revolver, semi-automatic pistol, shotgun, semi- and fully automatic patrol rifle, and submachine gun.