Jimro, not all of Sierra's bullets have lower BC's as velocity drops. Check out the .308" 180-gr. HPMK's BC shown below:
.475 @ 2800 fps and above
.496 between 2800 and 2200 fps
.494 between 2200 and 1600 fps
.494 @ 1600 fps and below
Bart B, I'm shocked no one caught you on this one. This BC is for the current 180SMK that shares the same BT angle as the 168SMK.
Too keep this apples to apples use the old 180SMK with a BT angle of 9* its BC was .505 and it didn't drop like a rock.