> So anybody who wants to shoot up the place can be sure to avoid checking in with all their guns in their briefcase

Yep, all these type of security measures do is harass law abiding people while doing nothing to stop the real threats.

> Can these detect disassembled guns?

I was thinking about this - the only part of a gun that must be metal is the barrel and the brass cartridge casing. Those are the first things I'd look for, assuming the system is capable of resolving objects that small.

... and before someone asserts it, yes, there are guns in existence with ceramic barrels, and there are guns in existence that shoot caseless ammunition. Neither are obtainable, and they wouldn't work in combination as one of the primary benefits to using metallic cartridges is that they carry a large amount of the heat away from the barrel when ejected.

There are a few more parts that have to be made of metal depending on the type of action used by the firearm. At a minimum, you'd also need a metal bolt/breach face (the part that supports the rear of the cartridge) for any type of action. I'm also fairly confident that any springs used (the firing pin/striker/hammer needs one unless you want to fire it with a handheld mallet) in the design would have to be made of metal as well unless there are some composites I'm not aware of that match the properties of spring steel.

Adding that metal to a firearm doesn't add that much mass but those parts would increase the profile of the gun due to their size/shape/area covered in the eyes of a image sorting algorithm.