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SIG took the lessons learned from the Tread line and the competition-bred DH3, then folded in the ambidextrous control layout from the MCX platform to build the M400 SDI X-Series. The result is one of the most complete factory ARs you can pick up without immediately reaching for the parts catalog. The lower gives you a bolt catch and release, safety selector, and mag release that work the same from either side, plus an ambi charging handle up top. Left-handed folks finally get a rifle that treats them like a first-class citizen, and right-handed users will appreciate how much faster reloads and malfunction drills become once you stop reaching over the receiver.
The 16-inch chrome moly barrel wears a durable FNC finish and a 1:7 twist that stabilizes everything from 55-grain plinking ammo up to heavy 77-grain match loads. A mid-length gas system smooths out the recoil impulse compared to carbine-length setups, and the hybrid flash hider and compensator on the 1/2×28 threaded muzzle keeps the sight picture flat during quick strings. The trigger is a genuine upgrade too. SIG's two-stage Matchlite Duo breaks clean with a flat blade that gives you a consistent finger position, so there's no aftermarket trigger swap sitting in your future.
The upper and lower are machined from 7075-T6 aluminum with lightening cuts that trim weight without giving up strength, finished in black hardcoat anodizing. The free-floating M-LOK handguard uses a steel clamp-style lockup that noticeably reduces deflection, so lights, lasers, and bipods can hang off any side without shifting your zero. A Magpul SL-K six-position stock rounds out the furniture with a compact cheek weld that works well behind an optic. The flat-top Picatinny rail ships bare, leaving you a clean slate for a red dot or LPVO. It comes with one 30-round polymer magazine and is backed by SIG's fully transferable Infinite Guarantee. This is a rifle that shows up ready to run.
Yes, and it goes beyond the usual ambi safety. The X-Series lower includes a bolt catch and release, mag release, and safety selector that all work from either side, plus an ambidextrous charging handle. The layout borrows heavily from SIG's MCX platform, so lefties get full functionality without any workarounds.
No, it ships with a bare flat-top Picatinny rail. That is a plus for most buyers since it means you are not paying for backup irons you plan to replace anyway. The full-length top rail handles red dots, LPVOs, and magnified glass with plenty of room for offset or backup sights if you want them.
A 1:7 twist rate favors heavier bullets, so 62 to 77 grain loads tend to shoot the tightest groups. That said, this barrel still handles common 55-grain range ammo just fine for practice and plinking. If you are setting it up for defense or precision work, heavier match-grade loads will get the most out of it.
Keep it. The two-stage Matchlite Duo is one of the better factory AR triggers out there, with a crisp break in the 4.5 to 6.5 pound range and a flat blade that promotes consistent finger placement. It is a big step up from a standard mil-spec trigger and one of the main reasons this rifle needs nothing after purchase.
| UPC | 798681664634 |
|---|---|
| Manufacturer | |
| Manufacturer Part Number | RM400-SDI-16B-P |
| Action | |
| ATF Type | MODERN SPORTING RIFLE |
| Barrel Finish | Black FNC |
| Barrel Length | 16'' |
| Caliber/Gauge | .223 REM/5.56 NATO |
| Capacity | 30 |
| Color | Black |
| Length | 36.8'' |
| Magazine Included | 1 x 30-Round |
| Model | |
| Number of Magazines | 1 30 rd. |
| Package Height | 4.3 |
| Package Width | 11.3 |
| Product Type | AR |
| Safety | AMBI SAFETY CONTROL |
| Shipping Weight | 10.0 |
| 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 |
| Thread Pattern | 1/2"-28 tpi |
| Units per Box | 1 |
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