http://www.google.com/url?sa=t&source=web&cd=3&ved=0ahUKEwjL4vGkwZfRAhUD9WMKHeYXBcMQFggfMAI&url=http%3A%2F%2Fhuntingwithnonlead.org%2FPDFs_Main%2FWI%2520DNR%2520Copper%2520Bullet%2520Study.pdf&usg=AFQjCNFyGVxgwRxh4lzPSs83pRcpeGBGVw

Granted this is media, but it jives with what I have seen.
I might also add many mention "eat up to the hole", "drop a weight", "shoot for bone", etc. Why do you suppose that is?