The answer is progressive improvements in hollow point function and BC. All the BD2 bullets are able to expand at 200 fps lower impact speeds than the original BD line. That is as good or better than almost all lead core and all other copper bullets. The BD 2 line also has had increases in BC which further extends their effective range. This allows the reloader to have non-toxic ammo that is reliably effective to 750-1000 yds at reasonable muzzle velocities and retain very high reliable penetration at close ranges without excessive and unnecessary meat damage, unlike many lead core and fragmenting copper bullets. In the SBD2 line the BCs are maximized for 1000 yd+ ranges, especially at high altitudes. This is achieved largely through longer more efficient low drag ogives which also have the effect of lowering the impact velocity for expansion to some degree. The SBD2 bullets are longer, and will benefit from being shot from longer actions, say 3.2-3.3" in length. They also do very very well in the new shorter fat cases such as Shermans, 6.8 Western and the older, but highly efficient WSMs and SAUMs.