Sexton 25-pounder SP Gun, Canadian War Museum, Ottawa. The 25-pounder Gun SP, tracked, Sexton was a Second World War self-propelled artillery vehicle based on an American tank hull design, built by Canada for the British Army, and associated Commonwealth forces and other Allies. It was developed to give the British Army a mobile artillery gun using their Ordnance QF 25-pounder Gun gun-Howitzer.