If the facts are as reported and they fight the charges in court it should be dropped or an acquittal. Maybe not in the initial trial but certainly on appeal. There is no such charge as "handling an assault rifle in public" so they're probably using "disorderly conduct" which requires INTENT or RECKLESSNESS AND does one of the "qualifying" things as listed below. ORS 166.023 disorderly does not apply as it only pertains to making reports.
Unless they obstructed vehicular or pedestrial traffic....note that they must obstruct it, not cause some ninny to be concerned.... they should be fine.
Also note that (g) is unlikely to apply as it is a right to bear arms in this state.
The other potential ORS they might try is "menacing" but that too is not relevant and is shown below. That charge requires INTENT to place another in fear of imminent serious physical injury....doesn't seem to apply.
163.190 Menacing. (1) A person commits the crime of menacing if by word or conduct the person intentionally attempts to place another person in fear of imminent serious physical injury.
(2) Menacing is a Class A misdemeanor. [1971 c.743 §95]
166.025 Disorderly conduct in the second degree. (1) A person commits the crime of disorderly conduct in the second degree if, with intent to cause public inconvenience, annoyance or alarm, or recklessly creating a risk thereof, the person:
(a) Engages in fighting or in violent, tumultuous or threatening behavior;
(b) Makes unreasonable noise;
(c) Disturbs any lawful assembly of persons without lawful authority;
(d) Obstructs vehicular or pedestrian traffic on a public way;
(e) Congregates with other persons in a public place and refuses to comply with a lawful order of the police to disperse;
(f) Initiates or circulates a report, knowing it to be false, concerning an alleged or impending fire, explosion, crime, catastrophe or other emergency; or
(g) Creates a hazardous or physically offensive condition by any act which the person is not licensed or privileged to do.
(2) Disorderly conduct in the second degree is a Class B misdemeanor. [1971 c.743 §220; 1983 c.546 §5; 2001 c.104 §55; 2005 c.631 §1]