I am not sure, maybe this suggestion was offered, but it will be great, if weapons can be reloaded one by one bullet. For example, shotguns or grenade launchers should be reloaded in this manner.
hmmm... after your explanation, I don't really see the reason why you would want this ability in the first place. The micromanagement would be pretty intense, and your results may be fairly unsatisfactory.
Lets take a shotgun for example, and consider it has three types of ammunition: Buck Shot, Heavy Slugs, and Incendiary Scatter Shot.
What order would you load these in? and why?
Lets just say you loaded them in the order above and had 6 shots loaded:
Shot, Slugs, Incendiary, Shot, Slugs, Incendiary
Now, the first target you find has low armor, so your Shot round works well. Nice. The next one is vulnerable mainly to fire (lets say a Reaper). Oops, I've got a slug loaded, what do I do? Unload it to get to the Incendiary? Fire it so the incendiary is up next? What if the next target is best hit by the Slug for armor penetration? etc, etc.
Maybe its better just to have the single shot weapon, like M249/M203 grenade launchers and load the appropriate round before firing (adjusting TUs costs to allow this etc).
Tl:Dr I don't believe your idea is practical in game play.