A rare bullet. I have only seen a very few.
Despite its slightly smaller diameter, it was intended for the .58 rifled muskets. Almost every bullet with a cavity was meant to expand. The most interesting and, to me, more distinctive feature of this bullet is the counter-tapered skirt. I tapers from the outside inward at the bottom.
I only ever found one of these. Unfortunately it was during a volunteer dig for the NPS at Antietam and I had to turn it over. Found in the Piper Orchard.
There are probably more of these out there...just lost in large bins of common 3-ringers. It's always worth the time to look through dealer's bins of bullets at the shows!