Firearms: Source is a team based first person shooter, rooted in the traditions of the Firearms mod for Half-Life. Firearms: Source brings the best aspects of Firearms to the Source engine, and then takes them a step further. You'll see a host of familiar weapons and maps, but you'll find the streamlined game play to be faster, focusing on aggressive, skilled play. From nearly the moment you spawn you can expect to be engaged in furious action.
FAS is a quasi realistic FPS mod for HL2. It embraces real life weapon and realistic locations while still mantaining a fast paced and arcade like atmospehere. The game isn't perfect, but it is one of the more fun and supported mods of HL2.