Trying to melee with a firearm when your character is crouched and has less stamina than required for the action, can cause issues with the ADS and fire functions.

1. Reduce your total stamina to the point where it's below the amount required for a firearm melee attack.
2. Put a firearm in your hands and enter crouch stance.
3. Hold RMB and Press F to attempt melee attack, the character will automatically stand up and the stamina bar will turn grey.
4. Change to a different weapon, for example a knife or your fists, and swing at anything.
5. Swap back to your firearm (or even a different one) and notice how you cannot shoot or aim down sights.
6. In order to fix the issue, you must drop some items so that you have enough total stamina to melee attack.
7. Once the required amount of stamina has been reached, the character will automatically perform the previously queued up melee attack.
8. Firearm will now function as normal again.