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Plenty of AR-15s get designed in a conference room. The M400 SDI X came out of the SIG Sauer Academy, where instructors run thousands of rounds a week and know exactly what breaks, what binds, and what slows a shooter down. That experience shaped this rifle from the lower up, and the result is one of the more thoughtfully laid out ARs in SIG's lineup.
The headline feature here is the fully ambidextrous control setup, including a bolt catch and release built into the SDI lower receiver that you won't find on a standard M400. Lefties get a rifle that runs the way it should without workarounds, and right-handed folks pick up faster reloads and easier transitional shooting. Add in SIG's two-stage Matchlite Duo trigger with a flat blade and a clean 4.5 to 6.5 pound break, and you've got a rifle that rewards good fundamentals straight out of the box.
The 16 inch cold hammer-forged barrel wears a 1:7 twist, so it stabilizes everything from light varmint loads to heavy 77 grain match ammo. A low-profile gas block feeding a mid-length gas system smooths out the recoil impulse compared to carbine-length setups, and the muzzle is threaded 1/2×28 with SIG's hybrid flash hider and compensator already installed. The free-floating M-LOK handguard uses a steel lockup design that cuts deflection way down, so you can hang a light, laser, or bipod on any side without pulling your zero.
A six-position Magpul MOE-SL stock handles length of pull, the flat-top upper is ready for whatever optic you're running, and the whole rifle comes in at a manageable 7 pounds. The Moss Green Cerakote finish is a nice change of pace from the sea of black rifles, and it's tough enough to shrug off real use. One 30-round polymer magazine ships in the box. Whether this is your first AR or the one you've been waiting to upgrade to, the SDI X earns its spot.
No, this rifle ships without sights. The flat-top upper receiver gives you a clean Picatinny rail to mount whatever red dot, LPVO, or backup irons fit your setup. Most buyers pair it with something like a SIG Romeo5 or a 1-6x scope depending on how they plan to use it.
It's one of the better factory options out there for lefties. The controls are fully ambidextrous, including a bolt catch and release built into the SDI lower receiver that standard M400s don't have. You can run the whole rifle from either shoulder without reaching over the receiver.
Yes. The 16 inch barrel is threaded 1/2×28, which is the standard pattern for 5.56 muzzle devices and suppressor mounts. It comes with SIG's hybrid flash hider and compensator installed, and that can be swapped for the mount your suppressor requires.
A 1:7 twist stabilizes heavier bullets really well, so 62 to 77 grain loads tend to shine. That said, it handles common 55 grain range ammo just fine, so you can train with affordable ball ammo and load up heavier match or defensive rounds when accuracy matters most. It's chambered for 5.56 NATO, so it safely runs both 5.56 and .223 Rem.
| UPC | 798681701216 |
|---|---|
| Manufacturer | |
| Manufacturer Part Number | RM400-SDI-16B-MSGRN |
| Action | |
| ATF Type | RIFLE |
| Barrel Length | 16" |
| Caliber/Gauge | .223 REM/5.56 NATO |
| Capacity | 30 + 1 |
| Color | GREEN |
| Length | 33.25'' |
| Magazine Included | 1 x 30-Round |
| Model | |
| Number of Magazines | 1 30 rd. Polymer Mag |
| Package Height | 4.0 |
| Package Width | 11.3 |
| Product Type | Rifle |
| Safety | Lever Action |
| Shipping Weight | 9.65 |
| 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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