The Whitworth gun was constructed on the 'built-up' principle, not by shrinking one tube over another but by forcing one over another by hydraulic pressure, which achieved the same result, ie the gun was 'pre-stressed'. The material used he called 'homogeneous iron', a form of mild steel. On the piece the breech was externally threaded and closed by a threaded cap which swung on a carrier pivoted on the right side of the gun. Firing was by friction tube inserted in an axial vent in the cap.