This is how automated reload works at the moment. When player presses R, the the engine is looking for mags of the same type as the one loaded in weapon then for the mag with most ammo in it. If found, mag is reloaded. If the current mag has max ammo, nothing is reloaded (even if animation is played).

So if you have SDAR and 2 types of mags 20 and 30 rounds, whatever mag is in the weapon, this type of mag will be continued to be reloaded. Think this is fine.

What is lacking is keyboard shortcut to cycle through different types of mags. So in case with SDAR: player starts with 20 round mag in weapon and will continue to use 20 round mags until finished before starting with 30 round. With Ctrl + R player can switch to using 30 round mags at any time.

If more sophisticated mag management is needed it is always available though the inventory manipulation.