Model: SIG MCX Caliber: 300 BLK SIG SAUER offers upper assemblies and caliber exchange kits in various barrel lengths. All NFA rules apply to the installation of these upper assemblies to lower receivers and caliber exchange kits to upper receivers. Upper assemblies and caliber exchange kits with barrel lengths less than 16 a**may only be affixed to lower receivers that (1) are not recorded as a rifle lower by the manufacturer and do not have a stock attached to the lower receiver or (2) registered with the BATFE as a short-barreled rifle (SBR).