Lee 40 Mag Carbide Factory Crimp Die sizes the cartridge while being crimped so every round will positively chamber freely with factory like dependability The adjusting screw quickly and easily sets the desired amount of crimp. Trim Length is not critical so this extra operation takes less time than it would if cases were trimmed and chamfered. A firm crimp is essential for dependable and accurate ammunition as it eliminates the problems of poor ignition of slow burning magnum powders.