I wonder how that would work...
If I understood you, the order you loaded your weapon with will be the order you fire those?
i.e.
You loaded GL with normal nade, smoke nade, normal nade.
If you fire 3 times, they will follow in the exact same order, and there's no way to change that?
Also, why would you need different ammo types loaded in ARs for instance? I can relate to GLs, and\or Rocket Launchers.
I do mean they get used in the order they are loaded, and same with magazines. I also meant that the placement of magazines in your inventory (top to bottom) would be the order in which they are used.
As for ARs, just like Stem was saying, customized all purpose magazines.
Say you're worried about fighting armored and unarmored opponents in the same battle, you load a configuration of rounds to suit that. Say, 5 AP (armor piercing), 5 HP (hollow points), 5 AP and so on.
Alternatively, you could load the first half of the mag with AP and the Second half with HP to focus on different targets within the same mag and maximize your effectiveness.
Or just load multiple mags, some with AP, and some with HP, and move them around your inventory accordingly.
And yeah they would fire in the exact order with no way to change it unless reloaded. Load 2 smokes in the GL first and come across an enemy, you better shoot those off and get to the good stuff quick. I don't really think there's a way to implement changing barrels on a GL in game.
Edit: Also forgot to mention it would make using nice expensive rounds cheaper. Instead of loading an entire mag with precious cap costing cap-a-motherfucker fuel, you could just place a few here and there throughout the mag to get some more -UMPH- without losing too many caps.