Simply go to a ammo box and hit M <Default Key bind settings< and then hit F1 and then you'll have your ammo replenished in just a second.

This may give you an idea of how it works in-game


If you have your Loadout Menu Key set to something else instead of the default key M , you can just press the button that you chose to open the loadout menu


For example

G+F1

Z+F1

K+F1


Once again, This does not work in hardcore game mode but for community servers, it may or not work.

In the last objective on any map sometimes the ammo crate is invisible and some times it isn't but you can do the same thing M+F1, it works even if the ammo crate is invisible or visible in the last objective.