My position is that anyone can own a cannon and shells, but you only get to fire it once before the feds step in.
Giving a naval cannon to the average person does not threaten humanity any more than giving them a gun or an LLM does. None of them are a panacea for anything.
A mass shooter wakes up in the morning and thinks: "I'll fire my naval gun once, then politely hand it to the feds..."
It seems completely unhinged for civilians to have naval guns. You could level an entire neighborhood with one of those.