Got a fix for this 'gun glitch' (I think).
Scenario: Gun fires one round then clicks, gun doesn't fire at all, mag shows ammo loaded but rounds do not chamber, mag will not unload from the weapon while in the inventory or in hand.
Fix:
1. Place gun on the ground.
2. Use bumpers to go to the gun, then use triggers to highlight the magazine.
3. Hold A on the magazine and use the bumper to drag it to the HAND slot.
4. Load/unload the mag or do what ever you need to do then PLACE IT IN INVENTORY (preferably into the backpack - more on this in a minute).
5. Pick up the gun into your HAND slot.
6. Bumper to your inventory and highlight the magazine and hit B.
***weapon should now function properly***
Why place mags in the backpack?
After you gun is functioning properly and you want to change mags by hitting Y, the weapon will absolutely not reload (assuming you have multiple mags) if they are not in the backpack.
What if you only have one mag?
Obviously you'll need to manually load the mag after you expend it. This is treading dangerously back into 'gun glitch' territory. You can get the mag out without issue from the inventory then place the mag in your hand to refill it, but once you are done it MUST BE PLACED BACK IN THE BACKPACK. If you select your gun whilst the mag is in hand, you've just re-glitched your gun and you need to repeat the steps listed above.
Hope this helps. :)