The Hague Convention of 1899 wasn't that specific, wich again opened up for the use of poison, gas, trenchguns, and different other horrible weapons during WW I.
Since WW I was the war to end all wars, nobody saw the need for a new convention before WW II broke out.
The Geneva Convention did a lot to put that thing straight, but alas, in some parts of the world it's just an unread pice of paper..
A little off-topic, but Norwegian patrol vehicles in Kosovo used to be equiped with at least one pump-action shotgun, but that was merely for disposal of stray dogs on the consern of rabies.