I've run out of ammo with infantry on a number of accosions or at least had to adopt ammo-conservation tactics. And no, the shortages weren't due to overusing Z-fire!
As CBO pointed out, 90 rounds last only 15 rounds if you fire 6 shots per turn in your own turn. But only half that if they also fire max OP fire in the oppo's turn! While that isn't likely to happen, a unit that is heavily engaged both in it's own and during the opponents turn can easily run out of ammo in a 10-turn firefight. That's about half an hour of firing.
I've had that happen in infantry battles with fairly open terrain and little armor and heavy support weapons (early or pre ww2 for example) where you pound away at each other from hundreds of yards.
Narwan