Plastics as strong as steel
Plastics are incredibly versatile. Though they have polymers as a base, it is incredibly diverse. So diverse it is actually a bit weird putting them under a single umbrella. You can have plastic that is great at moving a current surrounded by plastic great at insulating the electric wire.
In that vein you have plastics that are as strong as steel. I do not know of they show up on scanners, but one or two were promising in at least being much lighter than steel. This kind of compound seems difficult to make and mistakes easily lead to weak materials, but technology advances to reduce it to next to zero. And even if these are picked up by scanners we can use different plastics. If those aren't available you can add a replaceable tube that needs to be changed every x shots.
It doesn't seem a stretch to then just make very expensive plastic guns. Having them as plastic already eliminates the scanners, sort of. If you look at scanners you'll notice you can also identify a lot of other stuff that isn't metal. So unless you find a way to keep it on your person it might still be detected. So you need to change the shape to something inauspicious, or have it blocked ny something. Like putting it into a thick laptop where battery and electronics hide the true nature.
Do note that if you keep it on your person they can still check you on items. Many scanners show areas where there is something weird. I've been searched more than once just for having an extra layers of clothing that was positioned strangely. I'm guessing it has to do with tightness of clothing. A gun can stand out.
Simply do not get scanned
Why make a weapon that is undetectable and go through security? You run the risk of being picked out just from your behaviour, random check or sheer bad luck. At the airport I used to work even the employees need to go through a security gate. It is a little more relaxed, but still very thorough. That being said there are ways known not to get scanned and still get behind the gates. If you do it right you can walk into an airfield armed like rambo with rocket launchers, heavy machine guns and grenades without ever seeing a security gate. I'm not saying it is easy, but it is certainly possible.
Make weapons beyond the gate
It is interesting how they prevent weapons from coming through, but you can make weapons from stuff you can buy beyond the gate. They even sell things that unmodified can be used as weapons for stabbing or slashing, not to mention to just club someone. But someone once tried to prove a point you can go beyond this, making a bow and iirc even a crude crossbow from stuff available in the shops.
So if you want to ultimate concealed weapon, have none. You can be nervous, picked out of a line, random security checks and a freak disaster leaves you naked with the contents of your suitcase all over the floor. They won't find anything as you don't have anything. As long as you have your credit card you can build a weapon beyond the gate and do whatever.