Sparrows Lockpicks (previously) has just released an extremely clever practice lock called "The Revolver" that makes four locks out of a single core: "Pinning configuration starts with Standard pins at 12 o'clock (Marked with a small arrow) and then moves clockwise to Spool, Serrated and finally Mushroom pins. The end result is a lock that gets progressively harder to pick open."
The Revolver is configurable; you can make it easier or harder, and go up to six pins deep.
I often pick locks as a way to keep my hands busy or occupy my mind when I'm nervous or bored. I usually get out of practice, then work my way up through my 1-5 pin set of progressive practice locks. I'm looking forward to swapping that out for the Revolver!
The Revolver [Sparrows Lockpicks]