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..."