I bought some at Wally World which were, I believe, aluminum with a gold colored plating on the outside. They came in a small clear bullet shaped blister card and were advertised to give 25% more velocity. I don't know if the velocity increased that much or not but it went from subsonic with lead pellets to supersonic from my Diana mod. 48/52, or I assumed that's what the crack of the report signified, anyway. Sorry I can't remember what name they went by, but I'm an old guy so it's just lucky I remembered as much as I did.
Steve