That's exactly what the Courts refer to when a decision is handed down supporting a case like this.Would this case also be within the 4th Amendment, in your opinion? Obviously I have previously opined that he was morally right, legally wrong, but your mention of "reasonable" reminds me of the wording of that Amendment...
"The right of the people to be secure in their persons...against unreasonable searches and seizures, shall not be violated..."
Obviously the officer lacked any manner of warrant to seize any property, but would that qualify in your opinion? I am honestly curious at this point as this case is not as "foul-smelling" to me as others I have seen posted.
If an Officer walks up to you on the street, asks if you have a weapon without any suspicion that you've committed a crime, then takes it away, that would be unreasonable.
If your mental state is in question due to trauma from a head injury, you volunteer information that you have a weapon, the Officer temporarily disables the weapon by taking the ammo, that would likely be considered not only reasonable, but prudent, given the fact that people with head injuries often exhibit irrational behavior.