if have cover, (pistol, normal times) why

fire at ALL? all you will do is MISS his bobbing head (assuming he has cover)many, many times. All you have to do, really, is prevent the enemy's outflanking of your cover. So "bob out" with your ONE eye, at a DIFFERENT place each time, READY to fire, (ie, stay BACK from cover, enough to fully extend your arms. But do NOT stay exposed long enough to be able to make an EFFECTIVE shot (at a cover user's head) You don't have to hit him, so why risk staying exposed long enough to do so, and why waste your ammo? If he's moving to another place, fully exposed, in order to outflank you, you can THEN fire at him, with a realistic hope of hitting him.