generally speaking it's important if you need to buy some time to reload, or if you have a long rifle and are about to be in very tight quarters as a smaller weapon is easier to swing. most people use a gas pistol as their sidearm, however you can use an AEP or a small smg (MP7, TMP, Mac11, Uzi, etc.) you just have to bear in mind that whatever it is it has to be capable of semi-auto, and the larger your sidearm, the more weight you're carrying. did i miss anything brazzy?