Differences in bullet construction.
I am not surprised at the differences in load data for the different bullets. While they are both "cup-and-core" construction, they have much different cups.
I once used Speer data with Nosler BT bullets (not yet having a Nosler manual). Loads that produced 3,000 fps with 130 grain Speer bullets produced 3200 fps with the same weight Noslers. I am sure that the difference was pressure, rather than magic.
But my M77 showed NO signs of pressure. I think that says more about the construction of the Ruger than my skill as a reloader at that point in my career. ANyway, I figured a steady diet of 3200 fps 130 grain Noslers would soon eat the barrel of my .270. So I bought a Nosler manual and found that their loads for their bullets produced nearly the same velocities as Speer's data for the Speer bullets.
SL1
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