I've looked for liberal anti-gun forums before too, and have not had any success. My desire to find one, is not to bash opinions and beliefs, or to act as a troll.
A good debater knows what the enemy is thinking. I'd like to hear their arguments, and understand their position, so that I can make a better case for my position.
Troll behavior is not only ignorant, it is counter-productive. It serves to foster a sense of distaste for your position in the minds of those who's opinions are not yet set in stone. And those are the very people whom it is important to convince.
Maturity, Intelligence, and Honesty are the only ways to win an argument, no matter how many low blows and lies you have to suffer from the other side.