I think I found a bug or a cheat.
If you do a mission, then on the heroes in victory screen you press Esc, it will take you back to the mission, but advance you on a rank.
I have never heard this being described anywhere else before.
Have I found something new?
Here is a pretty rubbish video showing it
http://screencast.com/t/ZWI4OTFiY

If you consider that a cheat you must be wanting some fancy tombstones... because it isn't going to do much else =)

Being higher rank increases your firing rate and range.

Range is always welcome, but firing rate is already good.