I forgot about this post. Thanks for bringing it back up. I did go ahead and build the AR in 5.56/.223, but have decided to get another rifle for the antelope. Have been looking at the Savage Axis. .243 Will be the caliber, I have been reading and reading until my eyes are starting to bleed and can't find a better caliber for accuracy and range of bullet weights.
I know it isn't one of the higher end Savages, but it seems to get some good reviews. Plus I can still afford to put on a fare scope, buy ammo and still have a little money for gas and lodging.
The .243wssm AR uppers are just out of my class as far as funds go,and the same thing goes for any AR10 variants. 260 was on the list for a while but I have read that some of the long range guys can do it just as well with the .243 and ammo is far easier to come by.