[6] It has been used by various SWAT teams and militaries, including the Ukrainian Armed Forces during the Russian Invasion of Ukraine.

[10] The MCX is designed to deliver optimal performance with .300 AAC Blackout and an optional suppressor.

The SIG MCX RATTLER is a short-barreled rifle variant chambered in .300 AAC Blackout and 5.56.

It is intended to serve as a personal defense weapon, featuring a 140 mm (5.5 in) barrel, and comes with a Picatinny rail tail interface for attaching either a compact buttstock or a folding PCB (pistol contour brace).

[26] In February 2018, USSOCOM ordered upper receiver group conversion kits for the MCX Rattler in .300BLK for evaluation.

While the civilian variant is referred to as the "SPEAR LT", referencing the SPEAR from which it derives many of its improvements over the Virtus line, SIG Sauer refers to the military versions as simply the "MCX LT" or "MCX Lightweight".

[29] The defining features of the LT are its new, lightened, handguard and barrel design (borrowed from the SPEAR) ambidextrous bolt catch and release, proprietary SL muzzle device to allow for the use of SIG SAUER's quick detaching SLX and SLH suppressors, and new coyote brown coating (black options are still available for purchase).

[32][33][34][35] SIG Sauer was chosen as the winner on 19 April 2022, formerly designated as the XM5 then was later changed to the XM7 in U.S. military service.

Unlike the handguard of the MCX, which slides off after pulling the front pivot pin, the MCX-MR requires popping off two screws first.