Yes the 223WSSM is legal. It is significantly over the needed foot pounds of energy at 100 yards. It would not be my first selection to take on an antelope hunt but I'm sure it would work. If you do decide to take it, practice in the wind so you understand how much bullet drift you will have. I would also limit your shots to 250 yards due to the loss in energy beyond that range. Just my 2 cents... .243 is my personal minimum for antelope.