I understand, I know that original content made by an author, like the weapons you convert, are totally ok. What I'm saying is that there are also lots of Battle Field weapons converted to, and a royal ton of CoD player models.
I understand what you're getting at, I promise I'm not arguing with you, there's just always this grey area of modding. Some companies have flatly said they don't mind as long as its not for profit. Well... Really only 2 come to mind; The people who made Stalker said they love seeing people mod with the content they made, and MS has stated as long as Chief's model is used for only non-profit mods its ok. A friend actually had some really ridiculous luck with Gearbox, he emailed them about doing a Borderlands companion mod in Fallout, their legal department actually replied and stated, "We cannot rightly confirm that you are allowed to used our material, but as long as any projects involving our work are done without profit, we don't see any legal repercussions."
Either way, this is less about copy right and more about whether it'd be worth doing or not. I mean GTA4's engine has grown on me already, its a pretty game when you make a few changes, I just can't help but ponder if it'd make a pretty wasteland?