MCX-SPEAR LT 9″ UPPER 300BLK BLK

Description

The SIG Sauer MCX-Spear LT upper in .300 AAC Blackout is built for shooters who want a compact, suppressor-ready host without leaving behind the reliability of a properly engineered piston system. It suits the builder who runs a standard AR-15 lower and wants to move to a short-barreled, piston-driven configuration without purchasing a complete rifle.

The MCX platform uses a short-stroke piston rather than direct impingement, which means propellant gases are tapped and used to push a separate piston rather than venting carbon directly into the bolt carrier group. The practical effect is a cooler, significantly cleaner receiver interior and reduced heat transfer to the shooter’s hand over a sustained firing session. A two-position gas regulator lets you tune backpressure on the fly — one setting for standard unsuppressed use, a second for the increased backpressure that comes with running a suppressor, keeping cycling reliable across both configurations without replacing components.

The nine-inch barrel length sits at a point where .300 Blackout supersonic loads achieve a meaningful velocity while the overall package stays compact enough for close-quarters handling. Subsonic loads, which trade velocity for hearing-safe performance when paired with a suppressor, require complete powder burn in a short distance — a characteristic the .300 Blackout cartridge was specifically designed around. The Spear LT handguard uses a revised attachment method that eliminates the rotational play present on earlier MCX handguard designs, which matters when a light or laser is zeroed to the handguard rail and expected to stay there under recoil.

Because the MCX recoil system is fully self-contained within the upper receiver, the assembled rifle does not require an AR-15 buffer tube at the rear, opening the door to side-folding stock or brace configurations on the lower. The flat-top receiver accepts any standard Picatinny optic mount.