I would look into a 200 grain Shockwave and try to get 120 charge accuracy at least under it. Your rifle is made to do that. That way with practice a longer shot would be possible. Another one I might look into would be a 250 grain Barnes Expander. They seem to take large charges well, they hold true in flight, and anything they hit they tend to bring down. Again, practice with some different ones and see what happens. I have never shot a powerbelt over 100 yards so I have no idea what they will do at the longer distances.

You might also want to go to Percision Rifle and look at some of the QT's or Dead Center Sabots. Look at some of the lighter projectiles and see if you can get some information off the site as to charges and expected performance.

To be fair I should point outI never hunted antelope before, but I would guess they are not too hard to penetrate or bring down. The longest I have shot is 150 yards and that was with the Shockwaves. I do shoot Barnes Expanders but have never tried them long distance because the rifle I shoot them out of has open sights so I limit my distance. I am sure there are others out here that will have much better information then I have for you.

Good luck with your hunt....