You can get TO niche in a forum... just like you can a business. A broader target audience, especially for certain niches that can broaden their footprint, are better and will usually provide better results. Trying to fixate on a specific item/trend is a losing proposition usually.
As an example, which would get more traffic? A forum specific to the Thompson Hakwen .50 calibre flintlock rifle (a very popular one that is still in demand even though I don't think it's still in production) and it's use for hunting, or one that dealt with black powder rifles (flintlock, cap and matchlock) used for hunting?
Or take it to cars..... a specific body style Corvette, or a site that has areas for ALL of the Corvette line?