Couple of choices here:
The range that Texas Tactical uses is out on Hamilton Pool Road, and run by Crazy Richard. I'd skip it for sighting in the AR.
There's also "Austin Skeet Range" just BARELY east of Manor (closer than Austin Rifle Club), but I've never shot on their rifle range either (only skeet). I hear of some safety issues with this range, so caveat emptor.
Other options, if you don't mind indoor ranges, are Red's North, and Red's South. They are both 100 yard ranges, but you have to deal with the hassle of shooting indoors.
If you can find someone in the Austin Area that's a member of Austin Rifle Club, you can go and shoot on their range (just east of Manor), which is very nice. I'm a member, but unavailable on Monday
Here's a good
link with some info on Austin area, and surrounding area ranges. Hope this helps!
-Gloftoe