The 30/30 Bullet was an item that could be found in a drawer in Jed Stone's room in the Slaughter Gulch Saloon, along with the Pearl and the Bulb. Carnby uses the bullet by inserting it into the lock of a door in the Town Hall and used a hammer to blow the lock up.
At some point, Cobra did a karate kick inside a lock of his jail cell to open it.