Re: 223 loads
Ehh.... I've not used my gas gun for that far (16" V-Match carbine), but it really depends on what you realistically expect. Personally, I'd say 500yds is a bit optimistic for a .223 on game, even small stuff like prairie dogs. But to each their own.
A couple loads that I've came across over time that work really well (all were suggested for others, so I'd say they all work pretty well for others, but reduce your starting charges anyway, YMMV) are about any 50-52gr bullet, and 27.7gr of Win748, 25.5gr of H335, or 26 or so grains of Viht N133. I don't remember how many grains, but Ramshot Xterminator is excellent also. I believe Ramshot Tac and Viht N540 work well w/ the heavier bullets. One that I've heard of recently and am curious about is, curiously enough, called '10x' Can't remember who makes it though. Think it was Alliant. Supposed to be excellent in .223, .22BR, etc. size cases. Hodgdon Benchmark is highly recommended, as is Varget (minor PITA as it is not a ball powder, w/ that small of a case).
In summary, a good cheap load would be Speer 50gr TNTs (about $75-80 per 1000, last time I checked), Win748, H335, or Xterminator. Win748 works, but shoots a little dirty. Xterminator works well, and shoots clean and is in my experience, extremely temperature stable.