It's not a "stupid" question, TMM. But it is a strange one, in ways. Are you able to hit an 8 inch plate anyplace I choose to put it, out to 500, from a field position (not a bench) without a rangefinder, with the .308? I know VERY few who can. But, if you can do it, you can do the same with the .223. You need to check the ballistics tables for the .223 Energy out there, though....I find it anemic....though acceptable for Coyote. (Never tried it myself.)
The real limiting factor, as others have pointed out, is accuracy. 8 inches at 500 yards: that's just about 1.5 Minutes of Angle. From a field Position. That requires a world class shooter, IMHO. If you're intending head shots, make it 3 inches....that's about 1/2 Minute of Angle.....if you can do your part, few AR Variants will be up to your level of accuracy.
Rich