The amount of velocity difference per inch will vary with case capacity vs bore size, and the actual barrel length. When I got into the 7mmstw, I saw data listing about 40fps/inch difference, going from 26" to 24". The 300wby has a larger bore for similar case capacity, so the difference should be slightly less than 40fps/inch. The 6.5x300 may exceed 40fps/inch, because of the even smaller bore diameter, but then again, the 6.5x300 is less efficient, and that may effect the results.
Now as you cut the barrel shorter and shorter, the difference per inch should grow, because the pressure is higher when the bullet is 20" down the barrel than it is when the bullet is 26" down the barrel.
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