In all honesty in factory chamberings it's a Tikka thing - they have a very long free bore / jump. My Tikka 270 WSM had very very underwhelming velocities when I first started handloading, until I started to (very slowly) exceed book max using 2209.
However if you've reamed it out to 280 that shouldn't still hold except for a new barrel that is still being run in?
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