I am not sure if it's as the break in but maybe as they warm up. I loaded up some 55 gr Noslar BT's in my 243 and tried them yesterday. The velocity went from 3333fps, 3363, then 3385 fps.( mild load)
I noticed this with my 6.5 x55 when shooting at the TePuke Rifle Club the recorded velocities increased as the barrel heated up.
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