It's a simple question with, I suspect, a complex answer. Many people will say velocity, and when comparing a round to itself, this is correlary...but is it causative?
Bullet A will expand out to XXX yards....so many cap that as velocity induced. Surely it tracks.
Then we introduce bullet C, which has identical construction front of the cannelure to bullet A...but with the edition of much more weight and much higher BC.
Now let's assume that both bullets impact at 250 yards. They are both travelling at the same speed because the lower initial velocity of C has been offset by C having a much higher BC. However...bullet C is carrying 20% more energy...
...will bullet C expand more than bullet A, or will they expand the same, and will bullet C expand to further distances than bullet A, if their velocities are similar but the KE of C is significantly higher?
This question may seem unrealistic, but in the case of the 62 and 75gr gold dot in .223, it is infinitely germane to understanding the long distance terminal effects of these projectiles. ..
So again, which causes expansion in soft point rifle projectiles...velocity, or KE?