<P> Smoke balls also date back to the 17th Century, British ones contained a mix of saltpetre, coal, pitch, tar, resin, sawdust, crude antimony and sulphur . They produced a' noisome smoke in abundance that is impossible to bear' . In 19th century British service, they were made of concentric paper with a thickness about 1 / 15th of the total diameter and filled with powder, saltpetre, pitch, coal and tallow . They were used to' suffocate or expel the enemy in casemates, mines or between decks; for concealing operations; and as signals . </P> <P> During the First World War, shrapnel shells and explosive shells inflicted terrible casualties on infantry, accounting for nearly 70% of all war casualties and leading to the adoption of steel helmets on both sides . Shells filled with poison gas were used from 1917 onwards . Frequent problems with shells led to many military disasters when shells failed to explode, most notably during the 1916 Battle of the Somme . </P> <P> The calibre of a shell is its diameter . Depending on the historical period and national preferences, this may be specified in millimetres, centimetres, or inches . The length of gun barrels for large cartridges and shells (naval) is frequently quoted in terms of the ratio of the barrel length to the bore size, also called calibre . For example, the 16 "/ 50 caliber Mark 7 gun is 50 calibers long, that is, 16" × 50 = 800 "= 66.7 feet long . Some guns, mainly British, were specified by the weight of their shells (see below). </P> <P> Due to manufacturing difficulties, the smallest shells commonly used are around 20 mm calibre, used in aircraft cannon and on armoured vehicles . Smaller shells are only rarely used as they are difficult to manufacture and can only have a small explosive charge . International Law precludes the use of explosive ammunition for use against individual persons, but not against vehicles and aircraft . The largest shells ever fired were those from the German super-railway guns, Gustav and Dora, which were 800 mm (31.5 in) in calibre . Very large shells have been replaced by rockets, guided missile, and bombs, and today the largest shells in common use are 155 mm (6.1 in). </P>

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