The SIG Sauer M400 SDI X 5.56 NATO pistol is an 11.5-inch AR-15 style pistol built around a cold hammer-forged barrel and a free-floating M-LOK handguard (part no. PM400-SDI-11B-FDE). SIG Sauer runs it on a carbine-length direct impingement gas system with a low-profile gas block, and the barrel carries a 1:7 twist with a 1/2-28 tapered shoulder thread. The SIG 2-Stage Matchlite Duo flat trigger gives a predictable two-stage press, and every major control is ambidextrous. A steel lockup on the alloy handguard keeps the rail solid for lights, lasers and grips. It is a compact, capable AR for home defense, vehicle carry and range work.

Key Specifications

• The chambering is 5.56 NATO, the standard AR-15 cartridge for defensive and range use.
• The barrel is an 11.5-inch cold hammer-forged steel barrel, built for accuracy and long service life.
• Rifling twist is 1:7, which stabilizes the heavier 5.56 bullets used in defensive loads.
• The muzzle has a 1/2-28 tapered shoulder thread with a compensator/flash hider installed.
• Gas is a carbine-length direct impingement system with a low-profile gas block.
• The trigger is the SIG 2-Stage Matchlite Duo flat trigger, rated at 4.5 to 6.5 pounds.
• The handguard is a free-floating M-LOK and Picatinny alloy handguard with steel lockup.

Frequently Asked Questions

Are the controls ambidextrous? Yes. SIG lists fully ambidextrous controls: bolt catch and release, safety selector, charging handle and magazine catch.

What magazines does it use? It takes standard AR-15 magazines, and SIG lists one 30-round polymer magazine with the pistol.

Can I change the muzzle device? Yes. The barrel has a 1/2-28 tapered shoulder thread, the common 5.56 pattern, with a compensator/flash hider installed from the factory.

Buy the SIG Sauer M400 SDI X 5.56 NATO Pistol, 11.5-Inch

SIG built the SDI X line around the parts shooters usually upgrade first: a hammer-forged barrel, a two-stage trigger and ambidextrous controls. Shop it with our semi-auto handguns and order from our Lubbock, Texas gun shop for fast, insured shipping to your FFL.