Just my 2 cents worth: If you have any buddies that reload, I'm sure you could talk them into reloading the 6.5 WSM. The XP that Kirby built for me in this cartridge is among my favorite for long range hunting. It is capable of handling the chore you desire---deer hunting from 200--500 yards. I have been using 120 Nosler BTs. Hornady 129 SST would be a good choice also. Haven't had a chance to experiment with the 130 Accubonds but will do so in the near future.
The 284 is another good choice, as Ernie mentioned. However, this round needs to be handloaded to get optimum performance.
If you just had to purchase factory rounds I believe someone is manufacturing 6.5 x 284 ammo. Another 6.5 caliber that is ideal for your purpose. I haven't checked but I'm pretty sure this loaded ammo would be fairly expensive.
I have been shooting a 308 Winchester for many years. Please don't yawn. I know it's garden-variety, plain Jane, and vanila simple but it is an effective deer hunting cartridge. While I have never shot anything at 500 yards with it, I have taken countless whitetail and a bunch of other game in Africa, Asia, Europe, and South America. Only reason I made that last statement was to emphasize a point; it works, and it does so in handgun length barrels. There is also a ton of ammo available for the non-reloader. Admittedly, this may not be your best choice for a 500 yard shot compared to the others mentioned.
Lastly, the Burris 3x-12x would be a good choice for a handgun scope. Good luck and have fun.