Used to be that partially depleted clips would be considered used up after the end of the mission, unless you unload them. That's how it worked in old UFO defense. OXCE uses a more fair method to determine consumption: Basically, it tallies how much ammo was used and gives you that much likelihood that a magazine is spent. So for example, if 10 shots were fired from a 30 capacity magazine, there's a 1/3 chance the magazine is considered used up after the mission, if there's a sum total of 50 shots of that ammo type across the battlefield, the consumption is 1 magazine + 2/3 chance of another.