While discussion of our fundamental rights is important and connected to the issue, we are getting a bit afield from the OP, which was essentially, how do we respond to this...
Their argument is that such weapons are not needed by any civilian and are only tools of mass murder.
There are a number of ways to respond to this kind of thinking. NONE of them are going to change the mind of the fanatic "true believer", but might have some influence on the uncommitted 3rd party listener.
Some responses (in no particular order) are..
"only tools of mass murder." So, why is there something that is only a tool of mass murder in virtually every police car in your state??? AND ON THEIR HIP???
After all, the only real difference between detachable box magazine fed semi automatic rifles and detachable box magazine fed semi automatic handguns is their size. (power and range although important to us, are not big points in the discussion).
Handguns, by virtue of their size are easily carried and concealed, and are equally "only tools of mass murder", which is something the anti's hammer on, when they are going after handguns. TODAY the target is rifles, Yesterday it was handguns, and likely will be again tomorrow.
Semi autos have been on the market for well over a century, but now, TODAY, they are suddenly "only tools of mass murder".
IS every cop a potential mass murderer? Is mass murder part of their JOB??
Then why are they armed with tools of mass murder AT TAXPAYER EXPENSE??
"well, the police are different, they have training"...etc." ARE THEY?, Really??
A blue suit (or black or urban camo) and a badge doesn't make them any different than the rest of us, we are all human beings, (despite some people's opinions
) with all the potential for good AND evil found in our species.
Cops are NOT God, they aren't even doctors who think they are God. They are people who are doing a job. A low paying, dangerous, often thankless job, that I wouldn't do, but its still a job. This is not cop bashing, its just an honest appraisal of reality. Being trained, wearing a uniform, a badge, and having been "cleared" by background check(s) does not make you a immune from evil.
A "tool of mass murder" does not become anything different when it is in the hands of someone paid to carry it. They cannot have it both ways.
If it
is "only a tool of mass murder" in the hands of a civilian, then it is also one in the hands of the Police.
Question for those who know the CA laws, are not detachable mag semi rifles ALREADY prohibited?? What is the "bullet button" law if not that??
SO, they are going after all semis, and wording about magazines is just a smokescreen??
Remember we are dealing with people who do not believe there is any legitimate use for ANY firearm, unless you are wearing a uniform, and or a badge, in which case, they are necessary tools for protection.
Many of these people, seem intelligent, otherwise, and a number of them have money, and influence, and egos that tell them what they "know" is right, and anyone disputing that is making a personal attack on them.
(not just limited to the gun control issue)
anyway, this is just one avenue of response, there are several others.