At this point, the gun is really starting to take shape. Let's add a little more detail to finish it up.
A great way to add detail (without too much work) is to run loop cuts across the existing faces, and then scale these new edges in by pressing Alt + S. This gives the appearance of a panel line or cut between different the portions of an object:
You can also create one such cut, and then rip the center edges. This will enable you to create a second line or cut that doesn't go all the way across the geometry:
Here, we'll create some additional vents for fins by picking a series of faces on the upper barrel and eliminating them:
Then, we can pick the two edges at the top and bottom and fill in a face. With a series of loop cuts across the face, we can easily create our fins:
We'll continue the detailing by adding a series of basic shapes to our gun:
Then, using techniques that we've already covered, you can go ahead and detail these small pieces. Here's an example...