What really ticks me off here is not whether the author got his weapon facts wrong, or that the MC got three people killed because of trying to save ammo. Its that I hoped there would actually a creative outcome to the situation. They were fighting an overwhelming enemy and I thought there would be a clever solution to this, but no, its just "pull out a bigger gun". At this point all suspense is gone from the story, because no matter the situation I will always assume if things go downhill, the MC just has to get a bigger gun.