im not sure you have a problem at all mate. In response to the geneva and the earlier hague conventions which prohibit expanding bullets being used on humans in war, the brittish designed tumbling bullets. These met the regulations of the conventions while also being more destructive. They did this by making a bullet with compressed paper in the nose instead of all lead under the full metal jacket. Ill bet that if you cut one of your problem bullets that you will find it has a paper core in the nose. Let me know how you get on.