Don’t know about Oregon, but California has had “Fruit Inspection Stations” on the state lines of Nevada and Oregon for decades. “Got any fruit? Where you coming from?” If they think you might have anything, they check the trunk, etc. just like coming in from Mexico. One time they searched my car because they saw an Idaho road map. So looking for ammo will be no problem.
They searched your car because you let them. They do not have that authority.