Essentially, pistols are for fighting your way to your rifles. Rifles are for fighting your way to the vehicles so you can escape the insanely huge army of zombies eating their way through the countryside. If there are more walkers than bullets there is no fighting to be done, just running and shooting things in your way. Notice I said: “… in your way.” Turning around and shooting undead, which are not currently obstacles preventing your escape is a waste of ammo and will only cause you to trip and get eaten. Also, shooting from speeding vehicles generally results in a miss, particularly when only head shots count.