You can't move a firearm to a container using Shift + LMB if the container already contains one or more of that particular firearm.
Example: If I have a normal shotgun in my inventory or hotbar and find a chest with another normal shotgun, I cannot use Shift + LMB to pick it up. To get it I must move it by selecting it with LMB and then drop it on a slot in the inventory/hotbar.
This is true for the following situations:
- * Moving firearm from ground to your inventory.
* Moving firearm from inventory/hotbar to an open container.
* Moving firearm from an open container to inventory.