Nice models jhubbard
Your best bet is to find mods and forums relating to the game you're going to mod for. They will give you a good idea of what the poly limit is and maybe help with suggestions etc.
I think there are a lot of wasted polys in the gun there. Especially in the area below the trigger and on top of the gun. Also the barrel is nice and smooth but could you knock down the faces a little to lower the poly while not impacting the shape too much?
Some of the grooves and indents could be used to normal map onto a far lower poly model (unless this is what you're going to do anyways ) and give just as good detail.