Was shooting long today with a fellow who wanted to learn some basics. We established drops at 5,6 and 700 yards for his .308 Win. Rem. M-700 VS. Load is 46 gr. Varget, 165 gr. Nosler Ballistic Tips, not sure about case or primer. Scope was a 6.5-20 Leupold set on 20. Drops were 500 - 12 moa, 600 - 16 moa, 700 - 19 3/4 moa. Rifle shot well, had some three-shot groups around .5 moa. Five above zero C, winds were 6-8 mph from 11.30. His ammo was reloaded, not sure how many times for the brass used initially. He ran out of the stuff that had been reloaded, switched to reloads with new unfired brass. Shot at 100 yards and he hit into a previous group. Downrange drops changed to: 500 - 10moa, 600 - 15.5 moa, 700 - 19 3/4 moa. Why did the 500 yard zero change two minutes? Checked and rechecked - this is what the rifle and load did. How can the zero go out two minutes at 500 and virtually back to what we had originally at 6 and 700? Only variable was the unfired brass. Accuracy remained impressive, he had some very good 3-shot clusters at each range. Pretty confident this took place as described but not sure why.