just based on the job it as to carry out. It all comes down to range, weight, and velocity. The 14.5 and the 20mm are usually shot from mounted positions (ie. vehicles and aircraft) and are more of a "damaging", round, shot from closer distances, and shaped to hit with a broader surface, where as the .50 is designed to slice through the air for longer more accurate shots, and shaped to penetrate as easily as possible. Because at longer ranges the .50 will lose velocity, and power, so it needs to be able to do its job more efficiently.