The Mag version is a bit harder, either with a thicker jacket or harder core, not sure which (or both?). I guess at high enough pressure, the softer regular XTP swells out into the rifling more, causing higher bore friction. I don't have any proof of "how", that's my guess, just proof of the end result.
Not a bad 'guess', IYAM.
IIRC, back when I was using them ~15 years ago Hornady had a warning not to interchange them, not sure if that's still in their newest manuals?
I'm thinking I need to update my Hornady manual - it doesn't even mention the magnum 300 gr. bullet..
