Spinning the crap out of a bullet can help, by adding a lot of centrifugal force. That'll help the petals already want to spread apart.
The issue I've seen with bullets with small open tips and soft material is that the tip can be easily deformed and when thet happens it tends to compromise the ability for it to expand as designed. Adding centrifugal force can help like I said, but a larger opening, broaching, larger cavity, a tip and with a good interface that helps them work together, etc all help too.
It's very possible adding RPMs could have helped here. It's also possible it wouldn't have mattered.
I'm just adding to the discussion.