If the short mag BT bullets are shot in moderate distances (@75yds or less) and don't impact something like bone and hit only skin area(no ribs) They will expand very little if at all and will only leave small holes on both sides of a deer. There will also be very little trauma on the lungs as well.
He specifically mentioned the 150gr .308" ballistic tip.This bullet expands quite reliable on soft tissue even at .308 win velocities,so driving it at wsm velocities will result in it expanding even more,not less.Driving it too slow can result in inadequate expansion,but the only problem with driving it too fast,is that it may over expand and not hold together resulting in reduced penetration.I use lung shots myself so in many cases,no bone was struck,yet I have never had an issue with a ballistic tip not expanding.
If the short mag BT bullets are shot in moderate distances (@75yds or less)
The fact that you even mention 75 yards or less gives the impression that you think that the bullet did not expand was because it still had too much velocity because of the close range.Why else would you even bother to mention the 75 yards or less?There is a myth that some people still believe that a bullet won't expand if driven too fast.This is the myth to which I refer.The fact that you specifically mention the short mag BT puzzles me.The cartridge that a bullet is driven out of is irrelevent,it is the impact speed that determines bullet expansion,not the cartridge that it is driven from.