On The Range
After a few test loads were tried, the calibration load selected was the 135-grain Sierra MatchKing, 55 grains of IMR-4831, Winchester primer and new, previously unfired Winchester cases. During initial confirmation a series of six 5-shot groups were fired rapidly without cleaning the barrel or allowing it to cool. The purpose was to determine whether the barrel had any tendency to shift point-of-impact or lose accuracy when hot or badly fouled. If any of the six groups had been larger than 0.750 inches (¾-inch), the barrel would have been replaced. The largest group of the series measured 0.682 inches, with the average of the six groups being 0.624 inches. (Consistency is as much the essence of a good calibration load as accuracy.) The last group of the series, fired when the barrel was extremely hot and fouled, was one of the smallest, indicating we had a reliable test rifle.