The term BYGONE BRITISH GUN can be solved by the crossword solution STEN, which is a four-letter word. The STEN gun was a British submachine gun used during World War II. It was designed in 1941 and was mass-produced from 1942 to 1945. It was a simple and inexpensive weapon that was easy to use and maintain. The STEN gun was widely used by British and Commonwealth forces throughout the war and was also used by resistance fighters in occupied Europe. After the war, the STEN gun was phased out of service and is now considered a bygone British gun.