A couple of surprising Jack Johnsons

In the first months of the First World War there were references to slang terms for weapons in usages which seem at best thoughtless, and more likely shocking now, however innocent in intent may have been their use at the time; this is with hindsight, and they appeared before the horrors of trench warfare, the use of gas and the scale of the casualties were widely known. As the war progressed usages like these for recreational objects or activities became less common; the use by civilians (especially politicians) of any slang terms that had a basis in the experience of military combat, such as 'under fire' and 'over the top', were strongly condemned by G K Chesterton in the Illustrated London News 14 December 1918 . 

The Rochdale Observer on 31 October 1914 carried an advertisement for fireworks that read: 

Fireworks! Fireworks!

Ask for “Black Maria” or “Jack Johnson” shells 

Wholesale House :- Edwards & Bryning Ltd, ...

and the Western Gazette 11 December 1914 carried an advertisement for a box of 100 toys marketed by the Allies Toy Co in Brighton, which included a scenario described thus:

 Boom! – Oom! – Om! – M! – Bang!!! 

The “Jack Johnson” great German Gun is at work. First 25 harmless shells explode with a bang, then the Red Cross Nurses and their white Tents appear on the scene to deal with the wounded. 

The term is used here for the gun rather than the shell, a late occurrence for this usage, but not an isolated incident.