Notice the part I bolded above. The Oregon state law you posted is in direct conflict with the Federal law, and, therefore, the Federal law prevails.
Explanation: Federal law says that I have a right to transport a firearm through a state provided that neither the firearm nor ammunition is directly accessible from the passenger compartment, or absent an exterior compartment, provided that the firearm and ammunition is in a locked container.
Oregon state law states that I do not have the right to transport a firearm through Oregon unless the ammunition is in a separate container: a requirement which is not specified in Federal law. Therefore the Federal law nullifies that requirement in Oregon state law because of the conflict.
Don't believe me? Well, if that is true in Oregon, than we could simply take that one step further in a state like New York and simply further restrict the right to transport a firearm to those that obtain permits from the state to do so - and the state would only have to deny permits.
The very first words of the actaul statute in question, 18 USC 926A are: "Notwithstanding any other provision of any law or any rule or regulation of a State or any political subdivision thereof". Notwithstanding means despite.
The NRA disagrees with you.
" If in doubt, a traveler should carry firearms unloaded, locked in a case, and stored in an area (such as a trunk or attached toolbox) where they are inaccessible from a vehicle`s passenger compartment and not visible from outside the vehicle. Any ammunition should be stored in a separate locked container."
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