In eastern colorado, it is pretty open and hard to stalk. It is still possible with good planning to take a shot as close as possible. i would not plan on shooting at 500 yards, or you will take that shot with out trying to get closer.
give that antelope your best and I bet you can close that distance.
In some places here, antelope run away at 1 mile. Plan your hunt and stalk and 250 yards is pretty easy and an easier shot too. Maybe you'll get within 100 yards. Yes, it will take more time, but then almost any legal caliber will work.
I shot a muley 3 years ago with the 300 mag, and I won't do it again at 100 yards. It was devestating. Looked like a taco in slow motion, I had it filmed by a friend of mine.
I worked mine in in october from 1 mile to 246 yards. I wanted a closer shot but with shooting around me they were pretty spooked and it was not gonna get any closer.
I commonly shoot 400-600 yards at prarie dogs to the toon of 3-4000 rounds a year. Give that antelope your best, it deserves it.