Hydrogen and oxygen still need an energy source to get things going and despite what Hollywood likes to make us believe, a bullet is not an efficient way to blow stuff up.
For what it's worth, from Wikipedia:
Contrary to expectation, it was not easy to ignite the hydrogen using standard bullets and shrapnel. The Allies only started to exploit the Zeppelin's great vulnerability to fire when a combination of explosive and incendiary ammunition was introduced during 1916.
Since then, we learned how to use sparks to ignite gas mixtures, which is fortunate: tracer rounds would probably add a funny aftertaste to the stew.