Disappearing Ammo?

Why after being hit in the barrel and having to do a short repair, this it was about 12 seconds, why did I loose all my ammo? Surely if the hit was enough to destroy all my ammo I should have just blown up?

Depends, ammo rack explosions are not 100%. If the round does not have enough energy, or not enough damage in general was done to the ammo, the ammo will be destroyed but not cause an explosion. It is not common but it can happen.