A round that meets or exceeds that minimum in all test events will be more successful and effective over a variety of shooting incidents than one that meets the standards in only five or six test events. The expansion of each individual shot is measured by averaging its greatest diameter with its smallest diameter. Expansion can vary widely depending on the test event. For example, in test events 4 and 5, few rounds expand at all. Test Event 1 commonly yields the greatest expansion recorded.
Expansion is important in that it increases the size of the wound inflicted.
Given penetration in excess of 12 inches, the only means of increasing the effectiveness of the wound is by increasing its size. On the other hand, expansion can limit penetration unacceptably. Ammunition that achieves the best balance between these two factors will provide the best results within each caliber. Penetration in excess of 18 inches is ignored. It represents no practical benefit in FBI usage.