well the drop of a 223 would depend on the load but a factory load (remington's 55 hp) if zero at 100 it will drop 3 inches at 200 and 12.5 at 300 but all other loads will be different.
what you had happen at the range sounds like typical cold bore shot just reversed. in those temps your shooting your cold bore group by letting the barrel cool and your regular (hot barrel) group by shooting back to back. some guns have a big difference in POI in their cold and hot bore groups .
just a guess but it does sound right.
kinda why in most sniper matches the first class of the day is always CBS (cold bore shot) .
