The AR-15 is not a bad suggestion at all. With a Cavalry arms lower, you can get these down to 5 pounds.
Other options to consider that are a little more unique are the Excel Arms rifles. They make .22 Mag, 5.7x28 and .30 Carbine in a light handy carbine. The X-Series are the 5.7x28 and .30 Carbine. .30 Carbine ammo is inexpensive and the mags are plentiful. The new Speer Gold Dot round is a very nicely updated round.
Kahr/Auto Ordinance is also making an updated .30 Carbine.