MANY thanks for the responses!
Good news!! Verified original zeroes on a couple of previously dependable loads ( 69 and 75 grain ). Sub-MOA groups at 100. At 500, back to " normal " frequency of hits. Have no idea what the hell was going on yesterday, but in thinking about it, I do recall a bit of swirling breezes. It is , after all, a .223 and I didn`t make any changes because I didn`t think it significant at the time. COULD be. Anyway, scope ain`t broke! Thanks again, guys!