A loaded weapon within your inventory attempts to act like an ammo clip for an unloaded weapon in your hands. In other words, if you have a loaded Longhorn in your inventory, and are carrying an unloaded Longhorn in your hands - the game assumes that the loaded Longhorn in your inventory is a clip, therefore constantly prompting you to reload the unloaded Longhorn that is within your hands.
Pressing the 'reload' prompt, though, does nothing.
Tested with Longhorn only.
Screenshots -
UPDATE:
I was able to reload a Mosin Nagant by having a loaded Longhorn in my inventory. It simply showed me a reload prompt, I pressed it - and the game put the 1 bullet loaded in the Longhorn straight into the Mosin. (though the manual reload animation was played instead of the clip reload.)
Same thing happens to me with mosins all the time when I try to carry one loaded mosin behind my back and another one pick up with my hands.
Thank you for your report, TiredMiner!
I have confirmed the issue and it is now scheduled for a fix.
Regards,
Andy