There was an ambush assassination near the Empire State Building today, in which an individual took the life of someone he believed to be responsible for his misfortunes, and then was shot down in a hail of lead by police officers. I’m going to leave commentary on the actual incident to others.
What I will point out is that the onerous registration and permitting process to possess a firearm in New York City was defeated by buying a weapon twenty years ago and not talking about it since then. The requirement for a nearly-impossible-to-obtain carry permit, which the shooter lacked, was defeated by a layer of cloth.The vaunted perimeter security of the Empire State Building, including metal detectors and bag checks, was defeated by waiting outside the perimeter. Short of turning the entire country into a police state, and doing so twenty years ago, nothing could have stopped this person with murder on their mind from shooting their target.
It’ll get even harder to restrict access to firearms as small-scale machining and 3D printing become widely available. It’s about at the point today that you can create a rifle using a 3D printer and commonly available parts that can be ordered through the mail or bought at most gun stores; because they are replacement or customizing parts. The “serialized part”, the receiver, on an AR-15 is not an item that is subject to high stresses and therefore is suitable for manufacture in a 3D printer. This has been done already. If you have access to a CNC machine (and accessibility is becoming easier all the time), you can make an aluminum 1911 (the model of firearm used in this crime) or an aluminum AR-15. Note that complying with the odd restrictions of the federal Gun Control Acts makes this harder. If you didn’t care for the law (and if you’re going to commit murder…) a blowback operated smoothbore shortbarrel SMG is one of the easiest types of firearms to machine. (A breechloading shotgun is probably easier, and potentially more deadly for an assassination attack – and for the particular situation may have gotten about ten less people shot in the aftermath, depending). But a rifling setup is not impossible, just decidedly difficult.
It is impossible to stop a determined murderer once he has obtained his weapon and it is impossible to prevent the determined murdered from obtaining weapons under the US Constitution. There is no effective and Constitutional way to detect a concealed weapon in public. The most you can do is inconvenience the law abiding with ineffective and arcane restrictions.