Extra Magazines is a weapon modification in Battlefield Play4Free. It allows you to carry extra magazines. The amount varies depending on class. This option will give you the most extra ammunition of the available options in most cases, there are exceptions though. The AKS74U for example have a very small default mag, 15 round capacity only. In that case, the Casket-mag option is a lot better. There might be other exceptions as well so you'll have to look that up for yourself.
Extra Magazines adds :
+ 3 mags for Medics, Engineers and Recons
+ 2 mags for Assaults