I am not a lawyer or politician.
The first gun control laws followed an incident in 1980 or 81. A guy named Miller was released from the state mental hospital in Kaneohe. He caught a city bus to town, where he bought a .22 bolt action rifle and a brick of ammo from one of the sporting goods stores in downtown Honolulu. King, I think. He took that stuff on another bus to Waikiki, where the Royal Hawaiian Shopping Center was under construction. He went into one of the upper floors and started firing at people on the sidewalks. Hit five or six. All survived. Kalakaua Ave was shut down for much of the afternoon and evening. Police caught the guy fairly quickly, but had to clear the streets again when someone reported seeing a possible second sniper in the shopping center. This happened on a Saturday. I was there. It was before live television, CNN and the internet, so the story did not get as much attention as it would today.
I'm not sure about the history of legislation that followed. But I do know that when I bought a rifle before the Waikiki Sniper incident, all I had to do was fill out the federal form. There was another sniping case -- in a parking garage on Waimanu Street in Kakaako a few years later and other firearms crimes over the years. I think some kind of registration law was already in effect when the Xerox murders occurred.
At one point, I think in the 1990s, after mandatory registration was enacted, gun owners were advised to bring in the weapons they had obtained earlier and register them. If you didn't take 'em in, the state and city have no record of 'em. The only way the authorities would know about such a weapon would be if someone informs them or they happen upon it, for example, if you took it out in public or committed a crime.