This has been mentioned in Q&A and never posted on forums. unless I missed it. While I don't think this is so important, maybe only during PvP since PvE are usually static shield and armor and you can set before the mission like in EVE with right ammo and armor hardeners but feature can come in handy during PvP after you scan the enemy mech.

So here is how it can be done in my opinion:

This is the best solution for rearming between 2 ammo types - no need to go crazy and have 3-4 ammo types available even during PvP - not necessary. But 2 is ok and can come in handy and easier to implement.

The idea is to be able to equip an ammo clip or ammo stack into clickable module window. Say create another little window in mech interface that is accessible from inside outpost/terminal where you can equip secondary ammo and bind a function/script to a specific key - TAB will work fine. And have that new interface window linked to a button on modules window when outside terminal/outpost. Then if we want to change the ammo quick we just hit the TAB button and there you go, hit it again and its changed back to the original ammo.

Or make that window available in clickable modules bar like weapons themselves. Something like this:

/--------\~~/--------\
|ammo1|~~~~|ammo2|
\--------/~~\--------/

|=============|
|------WEAPON---|
|=============|

I click on ammo1 it equips ammo1, I click on ammo2 it unloads the weapon and loads ammo2. All you have to do is just combine the script (Unload) that automatically unloads the weapon with script (LOAD) and have a new script that checks which button was pressed and plug that ammo variable into a load script.

You already have a script that enables all the weapons for firing at once, by default its bound to a SPACE key. All you have to do is to bind a new combined script bindable to a key that unloads all the weapons at once and loads the secondary ammo into them.

If ammo type is not registered in database then it may be too complicated and defeats the purpose since the feature is not that necessary. But if ammo is registered based on type (missile, slug, bullets etc) AND dmg type (kinetic, chimical/kinetic, incendiary/kinetic etc) - then you can easily use these variables in script and make it to check for type and dmg of the ammo registered in that specific box and make the weapon load that ammo.

Again, I just read Q&A and this is not something I WANT, even though I have to agree it can come in handy in PvP SOMETIMES.