I am the farthest thing you can get from expert on antelope hunting, but I have shot a few, and I have had two or three six power Leupolds that I have used to shoot some deer and antelope, and I am going to say that I KNOW that using the 6X just as it sits on your rifle in NO WAY limits you to 300 yards. I know this because I have killed antelope at over 400 yards with a 4X scope with no problem.
Use a range chart taped to your stock, a laser range finder, and wind permitting that will take you to 500 yards quite easily, if you are capable. The aiming error with a 6X scope is going to be about 1/6 minute of angle (discounting parallax, which is another issue) and you can't even begin to hold that close under hunting conditions.
