I general, velocities published by ammunition manufacturers are fired in velocity and pressure test barrels. These are chambered with minimum chambers and a specified bore cross-sectional area and have a length that is within 0.010" of the specified bore length, which is most often (though not always) 24". If you don't have a 24" barrel, a tight chamber, or your bore is different in dimensions or length, you won't get a matching velocity.
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