It's not "cool" though. It's a war crime using such ammunition vs any target where such a bullet can hit anybody, per Haague conventions since 1899 - this particular part of those, to be exact. Any sort of explosive ammunition qualifies as "easily expandable within human body". UK ratified it, as did all other major countries of Europe. So firing those against any Zeppelin sure qualifies - there were people in and outside of Zeppelin's hull during any combat situation involving taking fire, since it was needed to patch up any bullet holes quickly. To be honest, i also am quite puzzled why exactly brits had to use those bullets vs Zeppelins in the 1st place. Stated purpose - to take out Zeppelins, - was easily and obviously doable without any new kind of machinegun ammunition, because Zeppelin is such a big and relatively static target. So you can just blow it up to hell simply flying an WW1 airplane over it and throwing couple contact mines right down. By hand. Heck, probably even simple Molotovs would do in good number. And that'd be available pretty much any moment, without any need to make new ammunition, without all those delays to file the invention, etc.