The Sig Sauer MCX SBR 5.56 NATO 11.5-inch is the First Responder model of Sig Sauer’s short-barreled MCX rifle (Sig Sauer part no. WRMCX-556N-11B-LT-SBR). Its 11.5-inch barrel uses a 1:7 twist, a rate that stabilizes the full range of common 5.56 bullet weights, including heavier projectiles. The rifle is semi-automatic, holds 30+1 rounds and uses a safety lever. Like the rest of the MCX family, it runs a short-stroke gas piston system. It is a compact 5.56 rifle for duty, training or home use.

Key Specifications

• Chambering is 5.56 NATO, the standard patrol rifle cartridge.
• Barrel length is 11.5 inches, a compact length that handles well in vehicles and buildings.
• Rifling twist is 1:7, suited to heavier 5.56 bullets as well as standard weights.
• Capacity is 30+1 rounds, a full magazine plus one chambered.
• The action is semi-automatic, one round per trigger pull.
• The safety is a safety lever, a familiar AR-style control.
• The operating system is the MCX short-stroke gas piston, which keeps combustion gas out of the receiver.
• It feeds from AR-15 pattern magazines, so duty magazines carry over.

Frequently Asked Questions

What magazines does this MCX SBR take? It feeds from AR-15 pattern magazines and holds 30+1 rounds, so common 5.56 magazines carry over.

Do I need ATF approval to buy this? No. As of October 9, 2026, ATF no longer requires Form 4 approval, registration or a tax stamp for short-barreled rifles. It transfers on a Form 4473 with a background check through a Class 3 (SOT) dealer. State and local laws still apply. Taking an SBR across state lines still requires an approved ATF Form 5320.20.

What twist rate does the 11.5-inch barrel have? 1:7, a fast twist that stabilizes heavier 5.56 bullets as well as standard weights.

Buy the Sig Sauer MCX SBR 5.56 NATO 11.5-Inch, First Responder

As of October 9, 2026, this short-barreled rifle transfers on a Form 4473 with a background check at your Class 3 (SOT) dealer, with no Form 4 or tax stamp; state and local laws still apply. Order through Sig Sauer‘s short-barreled rifles lineup and our Lubbock, Texas team will handle the transfer, and it ships fast to your Class 3 dealer.