Surface-controlled bullet firing

This percussion-type gun is accurately depth positioned by using a spontaneous potential or gamma ray log. A surface-controlled, electrically ignited powder charge fires a hollow cylindrical bullet into the formation at each sample depth.

Each bullet is attached by two retaining wires to the gun; these are used to retrieve the bullet and core. The wires have a breaking strength of approximately 1,800 lbf to release the gun from the core bullet, which prevents a stuck core resulting in a stuck CST sample taker.