I was talking about the stock itself, the back face its too squarish but since its for a game (if it's FPS) you hardly will see the stock itself when its equipped. So i guess you just need to round it a bit, changing a bit of it's shape.
As you can see in this image the stock is rounded up
http://sgcusa.com/images/supporting_...rkerized_C.jpg, you don't need to make it rounded like this, just get it less squarish.
Hope you understand what i mean
About your last question i'm not sure which number is a good tri limit for a game model, i'm sure someone can answer that soon.