Ar15.com is really popular. But i really do not like gun forums, since each one is loaded with know it alls, and each one has it's own clique of well established know it alls, each of whom tries to get new posters banned/discredited if a new guy doesn't agree with them.
I just had this happen to me on the Kel Tec forums, where i was basically called a mall ninja and then banned because i disagreed with a few of the regular posters(who were quite wrong IMO).
So....Pelican is my gun forum.
There is a firearms section on my forums, if anyone is interested in checking it out.
http://www.a-10.org/phpbb2/viewforum.php?f=23
I mostly talk guns here though.