I don't think it is bad form at all...providing you aren't trashing the other forums, trying to start a competition or forum war.
To do otherwise makes it seem like an overpossessive spouse.
You really want people to ONLY talk guns on ONE forum? B.S.
I swing by ARFCOM only on the weekends, I know Glocktalk is a popular shooter forum. Both of these sound like specialty forums but they are not. Now, CZForum and Rugerforum, those are brand specific. There are a ton of other gun specific or brand specific boards out there too.
I go to MNGUNTALK for local stuff, sometimes swing by TwinCitiesCarry, but the guy who runs that has developed some real ego issues and causes a lot of problems on other gun boards. Most states have state specific gunboards, probably the most well known and most visited by out-of-staters is CALGUNS (California) and PAFOA (Pennslyvania firearms owners association)
There are also ''mission" forums like Opencarry.org
I spend time on WeTheArmed, I believe TheFiringLine is back up.