David
That really depends on how close the guy with gun is standing away from you. A very skilled and fast knife fighter can be quite deadly within 2-3ft.

Especially if the gunmen does not see the knife. Years ago the NYC transit police found a gunmen minus one hand which was still holding on to the gun. Seems he was in the middle of a robbery on the train when some guy standing off to the side whipped out a 10" custom forged Bowie knife and severed his gun hand with one swipe. The police arrested both of them. Later one of the police ordered a bowie from the same maker

True story
Lew
http://www.youtube.com/watch?v=XQE4f...eature=related