The whole objective of employing FMJ is to enhance your chances of firing through surfaces. Simply said, it allows you to shoot past your cover, boosting your chances of injuring someone. The bullet's damage is not enhanced; rather, the bullet's penetrating capability is raised. As for causing more damage, yes, in terms of hole size, it does. Large holes are often caused by expanding bullets, which can be found in AR-15 rifles and similar weapons. Expanding bullets work by using gas pressure to expand after they have left the gun. This causes much more damage than normal bullets, which simply pass through their target. For example, if I were to shoot a water bottle with a.22 rifle loaded with 6 FMJ bullets, the bottle would only hold 5 ounces of water. If I used regular bullets, the water weight would be the same. This shows that FMJ bullets don't add any more damage than regular bullets, but they do allow you to shoot past your cover, which may give you an advantage in combat.