To begin, thanks for participating in, as Samuel Adams called it, "the animating contest of freedom".

A splendid idea! To refine your thought a bit, "...let's take the guns away...from the unstable and the violent criminal.

I like and trust our system of government. (It's been pointed out here that one of the greatest detriments posed by the incumbent GOP [great orange plague] is the effects on the federal court system.) I realize that New Zealand's system is more efficient than ours but efficiency can be the enemy of freedom.

New Zealand's legislature is sovereign and their Bill of Rights is just another law.
https://en.wikipedia.org/wiki/New_Zealand_Parliament#Sovereignty
New Zealand's legislators can, simply vote to add, change or remove parts or all of NZ-BoR. To employ some hyperbole in explaining, 61 of the 120 of New Zealand's legislators can vote to roast and eat the other 59 and there would be no other part of the government to overrule such a decision.

Part of my problem with what is termed "gun-control" is the terminology. I view control as a myth. In any free society, the effectiveness of "controls" depend on the time, effort and resources a criminal would decide to employ. I also point out the, to date, the Bath School disaster is the worst mass murder at a school. I believe a few were shot but 45 were killed and over 50 were injured. The Winchester Model 54 used was a bolt action rifle holding 5 rounds in a non-detachable magazine.

Our Constitution and Bill of Rights are not perfect. But they are the beginning by clear example of the best current means toward maintaining freedom.In my not so humble opinion, a Constitution incorporating a Bill of Rights is THE PATH from monarchy or tyranny to freedom. Ours was the beginning of the worldwide acknowledgement of that. It should not be lightly changed or disparaged and the Founders saw to that in its design.

Having said that I would point out that firearm restrictions are a bit like the CIA; the successes remain mostly unknown or at least untraceable but the failures become widespread news. It's obvious that the current background check system is flawed and somewhat inconsistently applied. Work is need. As far as confiscating guns without individual court verdicts or determinations, I'm not onboard.