Originally Posted by WiskyT
According to my old Hornady manual, the BTHP match bullets, Hornady's anyway. are made that way for two reasons. The HP makes the bullet longer for a given weight which increases the bearing surface. Also, the HP allows the base to be closed which means it's more uniform. A uniform base is critical to accuracy.
As for expansion, the match type bullets, according to Hornady, are not to be used for game as there performance can't be predicted. The Sierra BTHP match bullets apparently are destructive and are used by the military for this purpose with a nod and a wink in addition to their accuracy.
There are HP varmint bullets. They are designed to basically disintegrate on impact to minimize downrange hazards. Some of them are so fragile that they will fly apart in flight if they are shot too fast through a too high rate of twist barrel.
I had forgotten all about that - the fact that HP or SP bullet have enclosed bases.