The Sig Sauer MCX-Spear LT 9-inch Upper Conversion Kit (brand part no. UAMCX-300B-9B-LT-BLK) drops a complete .300 AAC Blackout upper assembly onto any MCX-Spear LT, M400, SIG 516, or mil-spec AR-15/M4 lower, turning a 5.56 platform into a short, hard-hitting subsonic-capable rifle in minutes. Sig Sauer builds the upper around a buffer-free dual-rod recoil system and a quick-change barrel design, so swapping calibers or barrel lengths doesn’t require a punch set or an armorer’s vise. The 9-inch barrel wears a 5/8-24 threaded muzzle with a three-prong flash hider that’s ready to accept a Sig SLH or SLX suppressor without an adapter. A flat-top optics rail and an adjustable, suppressor-tunable gas system round out a package built for shooters who want a genuinely modular short-barrel .300 Blackout setup rather than a second dedicated rifle.

Key Specifications

• Chambered in .300 AAC Blackout, giving full power at short barrel lengths with a wide range of subsonic and supersonic loads.
• Fitted with a 9-inch barrel for a short, maneuverable upper suited to a pistol or SBR build.
• Features a flat-top optics rail for mounting any standard Picatinny red dot or magnified optic.
• Threaded 5/8-24 TPI and fitted with a three-prong flash hider that’s ready to accept a Sig SLH or SLX suppressor directly.
• Runs a buffer-free dual-rod recoil system, eliminating the need for a traditional buffer tube and spring.
• Includes an adjustable gas system that can be tuned for suppressed or unsuppressed shooting.
• Uses a quick-change barrel design that allows swapping calibers and barrel lengths without an armorer’s vise.
• Runs a gas piston operating system that keeps carbon fouling out of the receiver for cleaner, more reliable cycling.
• Includes an M4 stock adapter that accepts folding stocks and braces using a vertical Picatinny interface.
• Compatible with MCX, M400, SIG 516, and mil-spec AR-15/M4 lowers, so it drops onto a wide range of existing builds.

Frequently Asked Questions

What caliber does this upper convert my rifle to? It converts a compatible lower to .300 AAC Blackout, using a 9-inch barrel.

Is it suppressor ready? Yes. The included three-prong flash hider accepts a Sig SLH or SLX suppressor directly, with no separate adapter needed.

What lowers will this fit? MCX, M400, SIG 516, and any mil-spec AR-15/M4 lower.

Buy the Sig Sauer MCX-Spear LT 9-inch Upper Conversion Kit

Whether you’re building a dedicated .300 Blackout pistol or adding a second caliber option to an existing MCX-Spear LT, this factory Sig Sauer upper delivers a genuine drop-in conversion with none of the fitting headaches of an aftermarket build. Sig Sauer engineers it to the same standard as the rest of the MCX-Spear LT platform, and it’s part of our full gun parts and accessories selection. Order it from our Lubbock, Texas gun shop for fast, insured shipping right to your door.