Hmm...try Recreational Warfare then. But if you don't think the BFG is not one of the included weapons, well, perhaps it's not the game you're looking for. The BFG is the highlight of RecWar. It also sports UT commentary for multiple kills, two-player split screen gaming, network gaming, two-player split screen gaming AND network gaming simultaneously, up to 64 (62 bots + the 2 players on one PC) players in one arena alone. Alotta mayhem, eh?
It only sports a Tank (massive armor, slow speed and low maneuverability over land and even lower over water, 360-degree weapon coverage), a Hovercraft (medium armor, medium speed over any terrain except walls and insane handling, forward-arc 180-degree weapon coverage), and a Jeep (low armor, full speed over land and medium over water and high maneuverability, straight ahead weapon coverage). No Heavy Tanks here. But there are advantages and disadvantages as you have recalled existed.
Oh yes, and this game IS free. Not installed though. Just unzipped. Nifty that.
But I do hope it is the one you are looking for. It's the only really fun game to play with lots of people.
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Join me one in all in the neverending search for the Great Game.