I'm not sure if slinging guns is all that effective to make it worthwhile to build a character around it. Why not just sling stones or bullets? Seems more effective to me and will also have better range.
It's all about the flavour and the theme of the character. I've made some pretty "suboptimal" characters for RP and plot reasons, though I try to make sure they can pull their weight during encounters.
One thing I think of when people talk "gun fu" is that the shooters are almost always in motion. Sometimes, they move even if staying under protective cover makes sense.
The Warlock's Shadow Walk power would really play into that. So would the Elf power Fey Step.
Yup, that sounds like what I have in mind. Presently the arcane version of the character is a warlock who will dual wield rods with passive effects.