SD changes at the moment of impact, and not as relevant as people believe for expanding bullets. It is relevant for solid bullets, which is where it was first used as a datum of comparison for African bullets.
My .444 with a 300-gr Hornady XTP will outpenetrate my .30-06 firing any premium 180-gr bullet into the same medium. The .30 caliber bullets are traveling much faster, and both the .444 and .30-06 have the same kinetic energy. The .444 has more momentum.
