I know this seems like a rudimentary question, but I can't seem to depress faces and vertices to look like they do in the picture. Here is what I am trying to model:
Here is my screenshot of the cartridge so far:
Every time I try to extrude, it makes weird distortions and just looks so crude compared to the uniform look of the original picture. How can I do this without causing these problems? Keep in mind I want this to remain solid, so hopefully that isn't complicating things :)
Here is the .blend 
