 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.