At the time the 1.1 inch was designed there was an old international agreement that explosive shells should weigh at least 400g (it was adopted in the 19th century to prevent explosive bullets from being used against people). In practice, this was abandoned in WW1 and no-one else took any notice of it thereafter, but it was still technically still in force. The 1.1 inch shell weighs just 416g.

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Tony Williams: Military gun and ammunition website