Cover against sight.
Not only can one take cover against projectiles, but against sight as well.
The problem with this — sometimes — can be that you loose sight of
your attacker, too. Rooms have dark corners and, depend on the situation;
you can use that to begin a surprise attack — don’t move to the dark if
the attacker is watching.
What is the best firearm?
The one in your hand, when you need one.