That’s the rifleman’s quarter mile. I’ve got a NM Garand and an FN FAL that’ll do it if I do my part. That’s the Appleseed standard.
You are NOT going to be doing head shots at 500 yards with iron sights. You will need an optic, if you have any chance of succeeding.
In the LRPR courses I've taken in the past 25 years, the rule of thumb is this -- 2 MOA is the best that can be expected in the field. So, at 500 yards this means 10".
I have shot sub 2 MOA objects at 600 yards with iron sights, but this was under ideal conditions.
At a 1000 yard clinic I completed in AZ the practical limit of the 308Win is 800 yards due to declining trajectory. With optics we were able to reliably his 20" steel at 600 yards -- that's greater than 2 MOA.
Anyone who tells me they can do head shots at 500 yards has no real field experience.