Sounds like some "OmniShock" rounds to me; these were wadcutters with a screw threaded into the front and the front halves of the bullet slit into 4 sections. The idea was, the steel screw would make them "semi-armor-piercing", and further, on impact, the screw would be driven back into the bullet, spreading the 4 leaves on the front out into little blades, something like an airplane propellor. I've got a couple of these rounds in my collection (with a cutaway I made), and I'll try to post a picture tomorrow.