So it's worth asking whether their suggestion has worked in the past. When armed civilians confront gunmen, who usually gets the better of it?
Studies don't show a clear trend, according to a report from Slate. Relatively few shootings are ended by armed civilians, and of those that are, the intervention most commonly involves retired or off-duty police officers. Some examples, also from Slate:
At a 1998 shooting at a middle-school dance...which left one teacher dead and three other people wounded, restaurant owner James Strand took out a shotgun and chased down the teen shooter before persuading him to give up his weapon.
In a 2006 school shooting, London Ivey, a school resource officer; and Russ LeBlanc, a former state trooper and driver's ed teacher, confronted a Columbine-obsessed attacker in North Carolina who was firing at cars outside the school and had killed his father that day. Armed with his pistol, Ivey ordered the student to drop his weapons, and LeBlanc (who was unarmed) put him in handcuffs.
A counter-example: During the Gabby Giffords shooting, at least one armed civilian nearly shot the wrong man.