Magnums (or revolvers, in this case) are high damage, high accuracy, low mag size weapons. The only primary weapons that have a smaller magazine are sniper rifles. They only carry 6 rounds, but make up for it with high damage, and high accuracy to help make sure all the shots hit. Hitting enemies in head is not too hard, due to the high accuracy, and it reduces the shots to kill, which is great considering the tiny magazine.