The Shortest Rifle Ever Produced, the SIG MCX Rattler. SPECS: Barrel Length: 5.5 in. Gas-piston system Cartridge: .300 BLK & 5.56 Stock: Folding PSB SIG Enhanced Trigger 1:5-in. twist rate Overall Length: 23.5 in. M-LOK attachments MSRP: $2,719–2,727 Designed with help from elite military units around the globe, the Rattler gives users all the fire and fury of an M4 in a sub-gun sized platform.