For an antelope optimized gun, people seem to favor the .25 caliber cartridges such as the .257 Roberts, .25-06, and .257 Weatherby Magnum. .243 is also good. .270 is pretty good, but more power than needed. I imagine the .240 Weatherby is pretty good in this application, but I haven't heard it mentioned in this context.

Of course, I was talking about optimized. The .30-06 would be perfectly suitable to antelope hunting. If you have a .30-06 and not one of the rifles above . . . and you aren't just looking for an excuse to buy a new rifle!!! . . . just take the .30-06 and shoot with confidence. The antelope won't know the difference. Out to 300 yards the .30-06 ought to shoot flat enough to bag an antelope. Beyond this range you are a pretty good shooter (or should be if you are choosing to shoot at these ranges) and don't need to be seeking advice about cartridges and shooting from total strangers on an Internet forum.