The term 'OLD BRITISH FIREARM' can be summarized by the crossword solution of STEN, which is a four-letter word. STEN is an acronym for the name of a British submachine gun that was used during World War II. It was designed in 1941 and was mass-produced from 1942 to 1945. The STEN gun was a reliable and inexpensive weapon that was widely used by British and Commonwealth forces during the war.