OK, I asked my ballistic calculator for the flattest factory loads for the 7mm Rem Mag and the .300 WSM. With BC’s, SD’s, and gr’s aside, it gives the best performance based of flatness. I can also ask for less drift or most energy as well. The calculator has no emotions or personal preferences on the matter.
Not sure where the guy you work with gets his info because it’s not for factory loads. Notice again they both are very close in their amount of drop and the .300WSM again has more KE. I also only ran this one out to 600 yards instead of the default of 1200 yards so you can see them a little better.
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With the 7mm Rem Mag compared to the .300 WSM instead of the 7 mm WSM. The guns will be different as the 7mm Rem Mag is a long action compared the WSM’s short action length. This may be a bigger difference that the performance of these to calibers.