Not disputing that this is inexcusable behavior by the baggage agents. But, bear in mind that the agents could have been contract employees or employees of a United Express partner like Skywest or Mesa and serve multiple airlines. Or, could have been TSA...

I've flown United with firearms many times and never have had an issue. Don't work for them, etc, but this could happen with any carrier. What station (or potential stations, if you connected) did this occur at? If I were you, I would definitely be contacting the station manager(s) there to complain.