Since i started playing, about 2.5 years ago, I have been using the same keybinds to aim my guns. I had both, "ADS Toggle" and "Raise modifier" set to RMB (right mouse button). This allowed me to hold RMB to ADS, like in most shooter-type games. But I could also go into the "Hip-fire" aim mode by either just clicking the RMB to toggle the "Hip-fire" aim mode, or I could double-click and Hold RMB to use "Hip-fire" aim mode and then release RMB to lower my weapon and stop aiming. Also, while using either of these aim modes I could click the mouse scroll wheel and that would swap between the two aim modes.
Since the recent update the ADS function appears to no longer be working.