By Bruce Finley Denver Post Staff Writer Spec. Crysti Cason, with the Colorado-based 3rd Armored Cavalry in Iraq, wipes down an M215 .50-caliber machine gun, with an M249 she calls “Rosie” at her right. Cason, 22, joined the Army at 17 and is one of the few Army women to emerge as a top gunner. This is her second tour in Iraq. Baghdad, Iraq - A small striped bird sang atop a palm tree as Spec. Crysti Cason sat beneath it wiping dust off her weapon: a .50-caliber machine gun that fires hundreds of bullets a minute.