Many times we experience a small variation in our zero from the last time we shot, and as reasoning snipers we try to explain it. I do it, too. We have an obligation to ourselves to ask those questions. I can tell you, however, that when it happens at ranges of less than 500 yards, the math won’t back up the conclusion that temperature-induced internal or external ballistic variations are significant factors. Keep looking in another area. Hint: It won’t be humidity, altitude or barometric pressure. These are variables best covered in another article.