I saw this movie on TV back in the late 60's. From the points in your post you obviously didn't grow up in the 60's or 70's like I did. Ray Milland was in a lot of sci-fi and horror movies such as "Daughter of the Mind," "Frogs," "The Thing with Two Heads," "X the Man with the X Ray Eyes," and many more. He was a good actor, but, "Panic in the Year Zero" had a pretty bad script. Even as a 10 year old when I saw it on TV, I thought the goofy storyline with the thugs that threaten the family was pretty cheesy. If I remember correctly, the teenage son was played by Frankie Avalon who was famous for his singing career as well as his "Beach" movies with Annette Funicello (I hated musicals, especially those 60's types with the stupid dancing associated with everything 60's
).
Also, back in those days California wasn't anywhere near as corrupt, liberal, or radical (those were the initial days of the Sexual Revolution which started the radical/liberal uprising in the US that caused our current problems with liberals/anarchists/communists that make up the current democrat party and PC power) as it is today. Regular title I guns as well as title II Machineguns/silencers were OK in California back then.