This is an idea for my Hrud, but it can be used for any aliens you like. It is very simple (apologies for lack of pics, bolters are too small for my camera):
1. Take a Space Marine Bolter.
2. Cut off the entire front grip, and file it down.
3. Cut off magazine.
4. Reverse the magazine so it curves towards the grip, and reattach it a little farther forward.
5. Use green stuff to cover the exposed clip in the magazine, and connect the handle and magazine.

And voila, you have a unique-looking alien gun. If anyone attempts this and posts pictures of it for me it would be greatly appreciated.