bullets introduced as the Mark VII.Y in 1916. Several incendiaries were privately developed from 1914 to counter the Zeppelin threat but none were approved until the Brock design late in 1916 as BIK Mark VII.K Wing Commander Frank Brock RNVR, its inventor, was a member of the Brock fireworks-making family. Anti-zeppelin missions typically used machine guns loaded with a mixture of Brock bullets containing potassium chlorate, Pomeroy bullets containing dynamite, and Buckingham bullets containing pyrophoric yellow phosphorus. A later incendiary was known as the de Wilde, which had the advantage of leaving no visible trail when fired. The de Wilde was later used in some numbers in fighter guns during the 1940 Battle of Britain. These rounds were extensively developed over the years and saw several Mark numbers. The last tracer round introduced into British service was the G Mark 8 in 1945, the last armour-piercing round was the W Mark 1Z in 1945 and the last incendiary round was the B Mark 7 in 1942. Explosive bullets were not produced in the UK after 1933 due to the relatively small amount of explosive that could be contained in the bullet, limiting their effectiveness, their role being taken by the use of Mark 6 and 7 incendiary bullets. In 1935 the .303 O Mark 1 Observing round was introduced for use in machine guns. The bullet to this round was designed to break up with a puff of smoke on impact. The later Mark 6 and 7 incendiary rounds could also be used in this role. During World War I British factories alone produced 7,000,000,000 rounds of .303 ammunition. Factories in other countries added greatly to this total. Military surplus ammunition Military surplus .303 British ammunition that may be available often has corrosive primers, given the mass manufacture of the cartridge predates Commonwealth adoption of non-corrosive primers concurrent with the adoption of 7.62 NATO in 1954. There is no problem with using ammunition loaded with corrosive primers, provided that the gun is thoroughly cleaned with hot flowing water after use to remove the corrosive salts. The safe method for all shooters of military surplus ammunition is to assume the cartridge is corrosively primed unless certain otherwise. Care must be taken to identify the round properly before purchase or loading into weapons. Cartridges with the Roman numeral VIII on the headstamp are the Mark 8 round, specifically designed for use in