so yeah, it appears that HWP builtInWeapon ammo are fully loaded regardless that you actually have the ammo-item or not in store. the clipSize set for the ammo-item will only work as a "limiter" for how many times you can shoot in a single battle.
this "limiter" behavior can be reproduced also for not-builtInWeapons (weapons that use fixedWeapon: true that is, like XOp's UAV), by removing the weapon ammo dependency and adding a finite "clipSize" to the weapon.
example XOps' UAV: if you remove the rocket ammo and add, say, "clipSize: 4" to the launcher item, it will work like that, meaning that ech time you go into battle, it will have 4 rocket.
in battle, once ammo are depleted (0 ammo), the weapon icon will disappear from the unit hand; but the hwp itself won't be removed from your possessions and in the next battle everything will be restored.
XOps you should be aware (and you probably are, but better be sure) that your STR_UAV_ROCKETS item only counts as 1 rocket ingame, despite "clipSize: 4"; so you have to buy 4 of them to fully equip your uav launcher.