SIG Sauer M400 Tread Coil — 5.56 NATO, 16″ Stainless Barrel, ROMEO5 Included

The SIG M400 Tread Coil is the only rifle in our AR range that arrives with an optic already mounted.

A SIG ROMEO5 red dot, a two-stage Matchlite trigger and a free-floating M-LOK handguard come fitted. That is the package argument.

What the SIG M400 Tread Coil Adds Over a Base Tread

The standard M400 Tread is a good AR with a single-stage trigger and an empty rail.

The Coil variant adds three things: the ROMEO5 TREAD red dot, the SIG Matchlite two-stage trigger, and a lightweight enhanced handguard.

Bought separately, a ROMEO5 and a decent two-stage trigger run several hundred dollars plus fitting. That is the entire value proposition of this configuration.

Why a Free-Float Handguard Matters

On a standard AR the front sight base clamps to the barrel, and anything you attach to the handguard pushes on it.

A free-floating handguard mounts to the receiver instead. Nothing touches the barrel.

Lean the SIG M400 Tread Coil hard into a barricade or load a bipod and point of impact stays where you left it. On a rifle with a fixed handguard, it will not.

SIG M400 Tread Coil Specifications

• Cartridge: 5.56x45mm NATO / .223 Remington
• Capacity: 30+1, detachable magazine
• Barrel: 16 inches, stainless steel
• Twist rate: 1:8
• Gas system: Mid-length
• Muzzle: 1/2″-28 threads, three-prong flash hider
• Trigger: SIG Matchlite two-stage
• Optic: SIG ROMEO5 TREAD red dot, included (CR2032 battery)
• Handguard: Free-floating M-LOK, lightweight
• Receiver: 7075-T6 aluminium, hardcoat anodised
• Stock: Magpul SL six-position collapsible
• Controls: Ambidextrous safety and magazine release
• Charging handle: Ambidextrous, aircraft-grade aluminium
• Weight: 7 lbs
• Country of origin: United States

SIG M400 Tread Coil Twist Rate and Barrel

A 1:8 twist stabilises the full useful range of 5.56 bullets — 55-grain bulk through 77-grain match — without forcing you to pick a lane.

Stainless is chosen for accuracy over service life. It typically shoots tighter than a chrome-lined bore and wears faster under sustained fire, which is the right trade for a range and competition rifle.

The mid-length gas system on the SIG M400 Tread Coil gives a softer impulse than a carbine-length system on the same 16-inch barrel.

Ammunition and Parts

The SIG M400 Tread Coil chamber is 5.56 NATO, so both 5.56 and .223 Remington are safe — a pressure distinction documented by SAAMI. A .223-chambered rifle should not be fed 5.56; this one takes either.

Standard AR-15 magazines and parts fit. One 30-round magazine is included.

Stock up on .223 and 5.56 ammunition before your first range trip, and browse the wider rifle ammunition range for match loads worth the 1:8 twist.

The muzzle is threaded 1/2″-28 under the three-prong device. Suppressors are NFA-regulated items requiring a separate federal transfer through the ATF, sold separately.

Where It Sits in Our 5.56 Range

• Bear Creek Arsenal AR-15 A3 — cheapest entry
• Ruger AR556 — 1:8 twist, no sights
• Smith & Wesson M&P 15 Sport II — iron sights included, 1:9 twist
• Seekins Precision NX15 — billet receivers, Timney trigger
• CZ Bren 2 — piston, folding stock, ambidextrous
• SIG M400 Tread Coil — free-float handguard, two-stage trigger, red dot included

Buy this one if you want to leave the shop with a complete, sighted rifle rather than assembling a package. Browse more rifles available online to compare.

FFL Transfer and State Restrictions

This is a modern semi-automatic centerfire rifle. It ships to a licensed FFL dealer in your state, not to a residential address.

Provide your dealer’s FFL details after checkout.

Several states and municipalities restrict semi-automatic rifles with detachable magazines, collapsible stocks, threaded muzzles or flash hiders. Magazine capacity limits apply elsewhere.

Verify legality at your delivery address before ordering. See our terms and conditions, or contact us about your state.

Buyers must be 18 or older federally for long guns, 21 in some states. Safe storage guidance is published by the NSSF.

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SHORT DESCRIPTION (above Add to Cart)

The only AR in our range that ships with an optic fitted. SIG ROMEO5 TREAD red dot, SIG Matchlite two-stage trigger, free-floating M-LOK handguard, 16″ stainless barrel with 1:8 twist, mid-length gas system, ambidextrous safety, mag release and charging handle, Magpul SL six-position stock. 7 lbs, 7075-T6 anodised receiver, one 30-round magazine. Complete and sighted out of the box. Ships to a licensed FFL dealer.

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💬 Before You Go — Kept the ROMEO5?

Running a Tread Coil? Leave a review — and tell us whether you kept the ROMEO5 or swapped it. That’s the question every buyer weighing this against a bare rifle plus their own optic actually has. And if you came looking for a configuration we don’t stock, tell us what it was. 🎯

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Please purchase and use firearms responsibly and in accordance with all federal, state and local laws. This firearm ships only to a licensed FFL dealer in the purchaser’s state and cannot be delivered to a residential address. Purchaser must meet the minimum legal age in their jurisdiction — 18 federal minimum for long guns, 21 in some states. Semi-automatic rifles with detachable magazines, collapsible stocks, threaded muzzles or flash hiders are restricted or prohibited in certain states and municipalities, and magazine capacity limits apply in others. It is the buyer’s responsibility to verify legality at the delivery address prior to purchase. Orders that cannot legally be transferred will be cancelled and refunded in accordance with our published returns policy. Not for sale to persons prohibited from possessing firearms under federal or state law.

Frequently Asked Questions

Is the red dot really included? Yes — a SIG ROMEO5 TREAD, fitted. The CR2032 battery row in the spec table is for it.

What’s the difference between Tread and Tread Coil? The Coil adds the ROMEO5 red dot, a two-stage Matchlite trigger, and an enhanced lightweight handguard over the base Tread’s single-stage trigger and empty rail.

Is the trigger single or two-stage? Two-stage. Any reference to a single-stage trigger on this page refers to the base Tread and does not apply to the Coil.

What does a free-float handguard do for me? It mounts to the receiver rather than the barrel, so bipod load and barricade pressure don’t shift your zero.

Can I shoot .223 Remington in it? Yes. The chamber is 5.56 NATO, which safely accepts both. The reverse is not true of .223-chambered rifles.

Why a stainless barrel? Accuracy over service life. Stainless typically groups tighter than chrome-lined and wears faster under sustained fire — the right trade for a range rifle.

Is it left-hand friendly? Largely. Safety, magazine release and charging handle are all ambidextrous.

Does it ship to my house? No. It must ship to a licensed FFL dealer in your state for transfer and background check.