This is actually starting to look really good. You just have to remember than any polygon with more than 4 sides is DEATH, and you have a lot of DEATH on your model. And as Snader pointed out, the piece above the barrel has a lot of loops on it that don't define any shape, so eliminate them.
Yep one important thing off the bat is to consider if the geometry added in will help with the shape and silhouette...if it doesn't like the excessive sides and three loops on the cylinder above gun barrel you should take them out...also makes it easier when you are ready to texture... I started looking into 3d when I was…