Why are there three different weights for the magazine?

When manipulating a magazine, such as while doing reloads, the weight of the magazine has a significant effect on the quality of movement. In other words, anyone who has reloaded with an empty magazine and switched to a full magazine will have noticed a considerable difference in moving the magazine to the gun. Therefore, to make training as real as possible, the magazine is fitted with three weights, one to simulate 10 rounds of 124 grain 9 mm rounds, another to simulate a full clip of 17 rounds of 124 grain 9 mm rounds, and a final weight to simulate 15 rounds of 180 grain 40 caliber bullets, all with the approximately same center of gravity as the location within the magazine. Of course, there are different bullet weights, but these ranges give a close approximation of those which most simulate a real magazine.