This is something you should get relatively simply through natural game play. Make sure you pick up any new weapon that an enemy drops since those count for this. There are a total of 30 throughout the game.
This 3-part guide will help if you're looking for a specific weapon or want to get all of them for yourself:
http://nemdam.hubpages.com/hub/Battlefield-Bad-Company-2-Wea...http://nemdam.hubpages.com/hub/Battlefield-Bad-Company-2-Wea...http://nemdam.hubpages.com/hub/Battlefield-Bad-Company-2-Wea...Credit: Nemdam