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FYI--Federal nickel plated brass and Hornaday brass weigh more than the Weatherby and Norma brass and could affect your handloads concerning the powder charge. Since this brass weighs more the wall thickness is greater and the internal capacity of the Federal and Hornaday cases are actually smaller than the Weatherby and Norma cases, thus the pressures are higher with the same powder charge. Anyway that's my recent experience in my loads for my 300 Weatherby and I had to uncork a bunch of loads with the Federal brass as they were hot, hot in my gun where as the Weatherby and Norma brass had no issues.
The human error when sighting in is always there from shot to shot. How tight and how you are you holding the gun, how tight do you hold it to your shoulder, are you 100% level are 3 things that can change shot placement from one to the next, if just one of them changes a bit your shot will end up being off from the other shots. Which begs a nice discussion on how do you sight in your guns?