Abstract: An improved caulking gun has a usual semicylindrical frame with a disc-shaped rear wall and a split front wall. A handle and trigger mechanism is mounted on the rear of the frame for advancing a plunger forwardly within the tube to discharge caulking material from the tube through the spout. A guide channel is mounted on and extends along the bottom of the frame and curves upwardly along and is spaced from the split front end. A flat strip of flexible metal is slidably mounted in the channel and has a front cutting edge which cuts partially through the plastic spout when a new tube is placed in the frame. The outer end of the metal strip blocks the flow of caulking material from the spout. A lever is pivotally mounted on the guide channel and is engaged with the metal strip for moving the strip between a flow-blocking position and an open material-discharge position.