bullet sizes

[rookie]

can someone explain the order of the sizes of bullets used in the glocks. and is there a norm for each size on when it is best used? do you nesscessarly want the biggest bullet? is there a specfic size that law enforcement use? stuff like that...

[BattleRifleG3]

If you're talking different cartridges/calibers, it's a question of accuracy, recoil, and capacity, which often just boils down to personal preference.
Rounds used in Glocks that are generally considered to be the most competent are 45 ACP, 40 S&W, and 357 Sig, in orger of heaviest to lightest, and slowest to fastest. 9mm is a step back in power, but is still commonly used, and offers the highest capacity available. To see how they all compare, check out a few ballistic charts.

Now if you're asking about different bullet weights and designs in the same cartridges, there's a lot of variety, which other people here know much more about than I.