Ammo in enlisted is handled in magazines/clips and currently refilling ammo at an ammunition box only replaces empty magazines/clips. In the following screenshot, I have taken this to its logical extreme, emptying all four of my 71 round magazines down to one bullet.
Despite having 4 bullets left on my person I am unable to refill ammo.
+1 from me.
Also a good suggestion on ammo sharing with squad.
I’d go a step further and permit scavenging of ammo from dead squad mates, or even enemies, if the ammo type is a match( would be if you take their gun too).