Sig Sauer MCX Rattler 5.56mm NATO 5.5in Black Modern Sporting Pistol – 30 1 Rounds – The SIG MCX Rattler was designed from the ground up to be as discrete as possible while retaining all the robust capabilities of the MCX. It features a 5.5in PDW barrel. Free-floating M-LOK handguards matched with a PDW upper and barrel.