The primary factor effecting how bullets expand is the resistive force that they encounter.This in turn is determined by the velocity of the bullet and the media that it impacts.Given the same media the faster a bullet travels the more it expands.If the media is bone or another hard media the bullet will usually expand much more than if the media was soft tissue.It is possible that a bullet moving faster may expand less than a bullet moving slower if the faster bullet strikes a media that offers less resistance but slowing a bullet down will not make it expand more if it strikes the same media.In other words a slower bullet striking a rib before entering the chest cavity may expand more than a faster bullet that slips between the ribs but if both bullets strike the rib or both slip between the ribs the faster bullet will expand more.The idea that a bullet is travelling too fast to expand is a myth believed only by those that do not understand how and why a bullet expands.