Apparently this is a myth. A book published in 2005, and reported in the Daily Telegraph, claimed firstly that there is no evidence that the cannons came from Sebastopol, and secondly that they weren't used until 1914. It also says that they weren't used between 1942 and 1945 because "the ingot" was mislaid. The Telegraph article doesn't really explain what it means by "the ingot" (I assume it means a chunk of metal from the cannons that was melted down to make the medals), or what has been used since 1945. It does imply that the original "ingot" was eventually found, and it says that there's enough metal left to make 85 more medals.
The cannons that were first used in 1914 are of Chinese origin, and where they actually came from is a mystery.
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