The "Next Recipe" contextual menu can be used for unloading ammo. Just insert/implement that menu for the loading ammo sequence/animation so that the option is added below the "Load Ammo" prompt. This can be also the alternative option to un-jam weapons and other actions you may add in the future. For the moment the needed "recipes"/options that come to mind are a)Load Regular Rounds, b) Load Tracer Rounds, c)Unload Ammo, d)Unjam Weapon

Reload button can be mapped to LT+X. X button is the default reload button for most games, but since we have a lot of needed actions with the X, LT+X is a viable option.

If you are running out of options on working this out with a dedicated button combo, it could be done from the inventory menu when we have the gun in hand. Eg. 1. open inentory menu, 2. put gun in hands, 3. prompt appers at the bottom "Hold ... button to empty ammo".