You have a couple options when it comes to using a .30cal. One, is the 7.62x39 upper. From what I've read, they are accurate with good loads (not the cheap wolf stuff). If you handload, I'm sure you can get it down to varmint accuracy.
My choice would be to step up to the "AR-10" platform (Armalite AR-10 or DPMS .308). You would have a lot more range, availability of good, quality ammo, and good accuracy. I have a AP4 on order, but I won't get it for another couple months yet. It shows a weight of a little over 8lbs, but I ordered it with the carbon fiber handguard, and I plan on having the barrel turned down. I should be able to get it to around 7 to 7.5 lbs. That wouldn't be too bad for a rifle that will travel a little.
Otherwise, you could always step up to the .50 Beowulf, .458 SOCOM (
), or the .450 Bushmaster. These all work on standard lowers and are good out to about 200 yards, maybe farther with a competent shooter.