Many squeaky-clean barrels shoot an inch or more off zero at 100 yards. Once fouled by a thin layer of carbon and copper, they become consistent. Test your rifle to determine if this is necessary.
While you’re at it, check for cold barrel/warm barrel inconsistencies, too. A rifle that regularly prints bullets off zero from a cold barrel probably suffers a bedding problem. Either fix the inconsistency or train yourself to remember how much higher your rifle shoots its first shot from subsequent shots.
And remember the angled spread of bullet flight: A 1-inch (1 MOA) group at 100 yards becomes a 3-inch group at 300 yards, and 1 inch higher than normal at 100 yards will be 3 inches higher at 300.
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