RR, thanks for offering an olive branch to the others but, in fairness, you were only one of many who "hijacked" the topic and that is kind of what makes topics fun around here. Many topics do change as the posts come in or develop into more as it were. Long Range hunting is a legitimate topic and would do well as its' own topic.

In answer to the OP's questions, about the effective range of the .257 Weatherby Mag, yes, it would be effective at 500-600 yards. I'm sure somebody on the site has likely hunted with one at those distances. Just from a ballistics aspect, it would definitely have enough power for deer and antelope at those ranges.

It's a bit harder to answer your question about scope power for those ranges since the amount of power you would think is necessary might be different from somebody else's perspective or needs. I think it was NoMercy who brought up another issue to consider which is the higher power adjustable scopes frequently have a higher power low end (I believe his example was 6.5 power on the low end).

If you have a buddy who actually owns a .257 Weatherby Mag, that would be an easy enough answer--simply ask to shoot it at those ranges with whatever scope he has on it and see if it's enough magnification. If that's not available, you may want to shoot whatever rifle of yours has the highest power scope at 500-600 yard targets and see what you think about the power needed. If you have a reasonable spotting scope, you could further check what minimum amount of magnification you think is necessary. Good luck in your quest--sounds like a fun one.