Ideally, a shooter would acquire rounds from a single lot or batch of manufactured ammunition, zero the rifle to that lot, and then use only that ammunition until it runs out. This ensures that every bullet is as similar as possible to the previous one, and assists consistency. However, this is rarely practicable. A sniper will enter information into a Data On Previous Engagement (DOPE) book, such as lot number, temperature, wind speed/direction, humidity and altitude. If the same conditions are encountered again, the data is available to assist in making an accurate shot. Most information gathering must start over if the barrel is replaced, as each barrel has distinct characteristics.