We definitely need this feature in game i.e. option to repack partially used magazines with same type of ammo.
For example, you shoot 10 rounds from one magazine, then you reload a full mag into weapon, then you shoot 20 from that and reload third.
Now, you have 2 partially filled magazines - one with 20 other one with 10 rounds in it. Then we could combine the two using this feature to make one full 30 round magazine.
It should be applicable to ammo belts as well i.e. that you can combine the belts to get one fully loaded belt or to use ammo from belt to load partially spent magazines.
There is a Magazine Repack Mod that enables this feature, however it is glitched for some reason and doesnt work after you load previously saved game.