I'm clearly out of touch with the semi-auto world, but I didn't even realize parts were readily available to build an AR in 7.62x39. That's pretty cool.
Caribou is right that ammo is dirt cheap and it's a better hunting round than .223. I love my SKS. Might have to add an AR in 7.62 to my ever-growing "want" list now...
There are a ton of calibers available for the AR platform now. The most common ones, off the top of my head:
There are barrels chambered for just about anything nowadays, though. 243 wssm to 300 win mag, even. It might be worth looking into if you aren't a fan of .223, but like semi-autos.