I have noticed that both the Magnum and FNX-45 sometimes randomly 'disappear' from your inventory. This will happen whilst you are logged into the server (as opposed to upon joining a server) and they don't fully disappear; they seem to just be 'hidden' and are unable to be used or accessed.

The inventory slots where the guns were being kept are inaccessible showing that the gun is still actually there, and upon re-joining the server the guns and inventory will return to normal.