Police in Norristown have the dragnet out for a man with a gun.   Investigators say he shot another man on West Marshall Street near Saint Francis of Assisi School Friday night.   The victim had to be flown from the scene by a medical evacuation helicopter and is in the hospital recovering from his injuries.   The suspect apparently fled the scene in a white Cadillac.   The vehicle was found on a nearby street but the driver could not be located.   Six shell casings were recovered from the scene of the crime.