I don't believe there are any anti-gun forums, because there aren't very many anti-gunners, in the same sense that there are a bunch of us pro-gunnies. The few, the psychotic, the antis, can make a lot of noise, especially if backed by one or two wealthy or well-connected patrons, and attract a few followers for a march or a meeting, especially if they promise free shirts, hats, and food, but that kind of support is mighty thin. Win or lose, to the antis, there is little at stake. For us the stakes are higher and more immediate.
If you want to mud wrestle with pigs, there are leftie forums out there that cater to the psychiatrically challenged, but arguing in a fools forum seems like a fools errand.