Instrument for measuring internal diameter of artillery gun tube

The invention discloses an instrument for measuring the internal diameter of an artillery gun tube. The instrument comprises a hollow cylinder, a relative displacement optical grating, a relative displacement device, a zero displacement optical grating, a zero displacement device and a measuring rod, wherein the relative displacement optical grating is arranged inside the cylinder and used for measuring the internal diameter of the artillery gun tube, the relative displacement device is fixedly arranged inside the cylinder and used for driving the relative displacement optical grating to move in the radial direction, the zero displacement optical grating is arranged inside the cylinder and used for calibrating the zero position of the relative displacement optical grating, the zero displacement device is fixedly connected into the cylinder and used for fixing the zero displacement optical grating, the measuring rod penetrates through the cylinder in the radial direction, and the two ends of the measuring rod