I concur with moonge; I almost would guarantee you that the blades opened, and closed upon extraction. Especially if you're shooting the head I think you are (probably the miniblaster, if it's 78 grains). That head will open on almost anything this side of a piece of notebook paper. I tore the side of a hen turkey's neck off with one, and turkey flesh is surely much softer than foam -- or deer, for that matter.

Discipline -- not desire -- determines destiny.