SIG Sauer M400 SDI X-Series 5.56 NATO AR-15, 16 inch Threaded Barrel, Free-Float M-LOK, Ambi Controls – Excellent
The SIG Sauer M400 SDI X-Series stands out with a long free-float M-LOK handguard, and fully ambidextrous controls. It pairs a 16 inch threaded barrel and 3-prong flash hider with a flat single-stage trigger, an ambidextrous charging handle, and a Magpul MOE SL collapsible stock. Made in the United States, this rifle comes in excellent condition with original box, paperwork, and two Magpul PMAGs.
Condition
Overall Condition: Excellent condition, showing only faint signs of any prior use or handling.
Bore Condition: Excellent - Clean, mirror finish, no signs of wear.
Bore Rifling: Excellent Rifling - Sharp lands and grooves.
Specific Condition Notes: The rifle's exterior shows only minor, scattered scratches and small blemishes visible upon close inspection.
What’s Included
- SIG SAUER factory cardboard box with model label
- SIG SAUER paperwork/owner's manual card
- Two 30-round Magpul PMAG magazines
- BCM M-LOK vertical foregrip (not installed)
Receiver roll marks include "SIG SAUER M400 SDI" with a stylized "X," and the rifle carries a matte black finish on its aluminum receivers.
A 16 inch threaded barrel terminates in a 3-prong flash hider secured with a crush washer. The long, free-float M-LOK aluminum handguard includes a continuous, T-marked Picatinny top rail with visible T2–T50 index marks, providing consistent reference points along the length of the rail.
Controls are modern and user-friendly: an ambidextrous safety with pictograms, an ambidextrous charging handle with extended latches, and a standard magazine release. The flat-faced, single-stage trigger offers a predictable break. The rifle is chambered in 5.56mm NATO with a 30+1 capacity.
The stock setup uses a Magpul MOE SL collapsible stock with an integrated QD sling socket and approximately 11–14 inch length-of-pull range. A SIG-branded polymer pistol grip complements the ergonomics. Overall fit and finish present clean lines and a durable matte black appearance.