This 20-year-old command is in need of tweakin'.. Edited for bump and clarifications.

Wiki simply says "Reload all weapons".
In fact this reloads the first muzzle it finds in currently held weapon that has had some ammo fired from it's mag (non-full mag), not for example the currently selected muzzle, even if it has completely empty mag if the main muzzle mag doesn't have full ammo.
So in multi muzzle weapons it just reloads the main muzzle if it has had ammo fired from it's current mag, always. Secondary muzzle stays untouched.

If main mag is full and additional muzzle mag is lacking ammo then it reloads the additional muzzle, but with a caveat:
When having a muzzle selected that is not being reloaded, like UGL on some assault rifle selected and rifle mag is reloaded (because how this command works), the soldier plays the reload hand gestures for the rifle mag reload but none of the related weapon animations play, as in magazine doesn't get removed/added back, charging handle doesn't move..

Going with the "Reloads all weapons" idea this should obviously "Reload all muzzles" then and actually reload the whole weapon system if needed. 🤔

Additionally, would be very very useful if you could add alternative syntax' for this:
<gunner> reload <muzzle>
<gunner> reload [<muzzle>, <magazine class>]

Where obviously the <gunner> would be soldier holding/manning the weapon and <muzzle> would the muzzle of the currently used weapon.
So that one could point the reload directly to a wanted muzzle.

The other one the same but you could also define the magazine class to reload with, obviously only if given mag is in gunners/vehicles inventory and can be used in the given muzzle.

I know this can be achieved with the reloadMagazine action to a certain extent but that's far from being user friendly as you need a complex-ish function to make it work with all the creator ID and stuff (and dunno if that's even 100% working solution).