Re: 500 yard load for 223 rem 1 in 9 twist
The 223 will easily make varmint kills out to 500yds. Your best bet would be to find the most accurate bullet regardless of weight and shoot for the record on a windless day. If that bullet is a 69-75gr bullet than you may have the best of both worlds.
This past groundhog season, I shot a few small groundhog pups @ 400yds with a .221 and a Nosler 40gr B-Tip. Naturally, I only use this set-up when the wind is down. When I had this rifle built, I figured that it would only be good out to 250yds or so. However it easily groups .5" @ 100yds and about 3" @ 400yds when there is little wind. I only have a Leupold 3X9 Compact scope on this rig and that is plenty for 400yd shots. Although, I did have Premier Reticle put 3 tiny aiming dots on the verticle crosshair.
For your first shot at getting into the 500yd Club, pick your days. Good luck.