Rarely does shooting alone destroy an army, but it can thin the enemy's numbers considerably, causing regiments to lose their nerve. When they do, they will often fall back and, sometimes, turn tail and flee.
If, during a single Shooting phase, a unit loses more than a quarter (25%) of the models it contained at the start of that Shooting phase, it must immediately make a Panic test. To make a Panic test, test against the unit's Leadership characteristic. If this test is failed, the unit has succumbed to panic. If this test is passed, the unit remains resolute and does not panic.