I don't know if this is intended, when you put a gun you had in inventory (for exampe MP5) in your hands, it is removed from inventory slots. When you pick up anything (for example can of food) it might take the slot which was used by the gun before. After that, when you press X to get the gun back to your inventory, it is dropped to the ground. So you constantly have to make sure, you have the inventory slots for the gun free. It would be much better if the inventory was prioritized, so when you pick up a can of food, it gets into other free slots in inventory, not the one preoccupied by the gun. I know you can work around this by putting the gun on the bottom of the inventory, but then it is harder to put it in the hands because you need to scroll down in inventory to find it.