a FN P90 OR a HK MP7? (light machine guns?)
p90 would be easier to model(though it d be cool if it had scope and silencer):

consisting from this parts:

HK MP7:


And, hands would be pretty cool! And as for ego shooters, maybe you can make a very low poly world models of the weapons too?
i mean like in css, a high quality view model with hands for firt person view, and very low poly(with 128x128 or 256x256 texture) model world.(drops when you die, the ones you see on the ground&pickup from ground)