I'm thinking a assault rifle caliber weapon, such as a .223 in a IW that's no heavier then four pounds, and of a bullpup design with a carbine length barrel(although it could easily be made longer).
I know magpul is designing a weapon with a 12.5inch barrel, which is basically what i'm talking about here. This would do away with a large amount of weight, but it would not cause to many logistical problems, as it uses ammunition commonly available, yet would still allow for more effectiveness at longer ranges.


Note: Please bear with any mistakes, grammar or otherwise, that I make. I am only 14