Honestly content is the biggest thing.
Looking at your site, you really haven't got a lot and you have made the classic rookie mistake of making too many forums when there's nothing to go in them. 50% of your forums are empty. If you can't think of something to write in them then your members won't be able to either. Cut it down to 6-8 forums in total, max. You only need more specific sections when you get enough posts on that subject to warrant a new forum dedicated to it. 1 forum with 100 posts looks better than 100 forums with 1 post each.
For example, you have 8 forums on long guns and hand guns with a total of 5 posts. Just have one forum on each, or maybe even one forum called long and handguns and then if you get a lot of posts on handguns, make a new forum for that. In fact you have Sixteen forums on guns and only 25 posts in them. Cut it all down. Be ruthless. It'll be better in the long run.
Making the site look busier with fewer forums will help your site develop much more than any SEO will.