If you want something fun, but not really skirmishable, try a Dan Wesson revolver.
They're CO2 powered and chamber 6 shells which each take a single bb in the front, pretty much functions like the real thing.
Nice build quality, good weight, feels solid as anything and looks very cool.
Unfortunately don't own one myself but had a go at shooting downrange with the 4" version a couple weeks back with some metal bb's.
From what I remember although you can get low power shells to make it useable on the field (standard ones shoot very hot) they don't use 6mm rounds, making sourcing plastic ammunition quite difficult.
Edit: On the other hand though I may be completely wrong, unless wolf armouries are incorrect they do fire 6mm. http://www.wolfarmouries.co.uk/airsoft/bb/product.asp?page=product&id=17351