Here is one I noticed no game does even the realistic ones. Why don't games, especially on more realistic FPS, have the player loose the bullets remaining in a magazine when they swap them?
The problem with this is that it’s not very playable. It’s not easy to intuitively explain to the player why the remaining ammunition store is going down for reasons that aren’t shooting those bullets, and it can become a headache trying to keep track of all of the rest of the data. The natural question that arises from losing the remaining bullets in a magazine becomes why the other magazines in the game are all full. You could try to store the number of magazines, with individual bullet counts per magazine, but then you have to track all of those as well. Imagine this scenario:
You’re playing as a soldier, and you find a new weapon. How many bullets does the gun come with? Will there be magazines with it, or just what’s in the chamber?
You find a box of ammo, which contains six magazines. Are the magazines all full? Do you need to have the player spend a few seconds refilling all of the magazines to full?
When you are reloading in the middle of a fire fight, do you need to select a specific magazine? What if you grab one and it happens to be almost empty?
The main issue is that there’s usually simulation for simulation’s sake, and there’s simulation for streamlining so that the player can get to the fun part. Tracking ammunition carefully like this is a lot of engineering hassle and doesn’t really add a lot to the game, which is why it usually isn’t done. I could see a feature like this specifically added for a hard core realistic sim, but we don’t have many of those. AAA shooters are more about the action movie experience than they are about simulating reality.