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Most AR builds turn into a months-long parts hunt. You buy the rifle, then start shopping for an optic, then a light, then a foregrip, and somewhere along the way you've spent more time browsing than shooting. Sig skipped all that with the M400 SDI Sentry. This is the M400-SDI X platform with a ROMEO5 Gen II red dot, a Foxtrot-MSR full-size light, and a vertical foregrip installed at the factory. Sig developed the setup with input from instructors at the SIG SAUER Academy, and it shows in the choices they made. Nothing on this rifle feels like an afterthought.
The rifle itself runs a 16 inch chrome moly barrel with a 1:7 twist, which stabilizes everything from light varmint loads to heavy 77 grain match ammo. The mid-length direct impingement gas system smooths out recoil compared to carbine-length setups, and the two-stage Matchlite Duo flat trigger is a genuine step up from the gritty mil-spec triggers you find on most factory ARs. Controls are fully ambidextrous, the free-floating M-LOK handguard uses a dual clamp locking system, and a 6-position Magpul MOE-SL stock handles length of pull. Overall length runs 36.5 inches extended and 33.25 inches collapsed, with the whole package weighing right around 8 pounds.
The ROMEO5 Gen II gives you a crisp 2 MOA dot with over 40,000 hours of estimated battery life, thanks to Sig's MOTAC motion-activated tech that wakes the optic when you pick up the rifle. It's IPX7 waterproof with fully multicoated glass, and the top-mounted controls work the same whether you shoot lefty or righty. The Foxtrot-MSR light pumps out 1,250 lumens and 33,000 candela, plenty of throw to identify what's at the end of a dark hallway or across the yard. A 1/2×28 threaded muzzle wears Sig's flash hider-compensator, and one 30-round polymer magazine comes in the box. Whether it's for home defense, training classes, or weekend range trips, this one shoots the same day you sight it in.
Yes, the ROMEO5 Gen II comes installed from the factory, along with the Foxtrot-MSR light and vertical foregrip. You'll still want to confirm zero with your preferred ammo before relying on it, but there's no mounting or accessory shopping required. It's set up to shoot on day one.
The Sentry is built on the upgraded M400-SDI X platform, which adds fully ambidextrous controls, a two-stage Matchlite Duo flat trigger, and a dual clamp free-floating M-LOK handguard. On top of that, it ships as a complete combo with the red dot, weapon light, and foregrip pre-installed. A base Tread comes as a bare rifle you'd need to outfit yourself.
Yes. The chamber handles both 5.56 NATO and .223 Remington safely, and the 1:7 twist barrel stabilizes a wide range of bullet weights, including heavier 69 to 77 grain loads. That flexibility makes it easy to run cheap plinking ammo at the range and switch to premium defensive loads at home.
Sig rates the ROMEO5 Gen II at over 40,000 hours of estimated runtime, helped by the MOTAC system that powers the dot down when the rifle sits still and wakes it instantly when you pick it up. In practical terms, you can leave it on and forget about it. The optic is also IPX7 waterproof rated.
| UPC | 798681716128 |
|---|---|
| Manufacturer | |
| Manufacturer Part Number | RM400-SDI-16B-R5FX |
| Action | |
| ATF Type | RIFLE |
| Barrel Length | 16" |
| Caliber/Gauge | .223 REM/5.56 NATO |
| Capacity | 30 + 1 |
| Color | Black |
| Length | 33.25'' |
| Magazine Included | 1 x 30-Round |
| Model | |
| Number of Magazines | 1 30 rd. |
| Package Height | 4.0 |
| Package Width | 11.5 |
| Product Type | Rifle |
| Safety | Ambidextrous |
| Shipping Weight | 10.9 |
| Sights | No |
| State Restriction (CA) | NO DIRECT SHIP TO CALIFORNIA |
| State Restriction (RI) | NO DIRECT SHIP TO RHODE ISLAND |
| State Restriction (WA) | NO DIRECT SHIP TO WASHINGTON |
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