[core-utility]

Only illegal in certain states, not federally

[PrivateButts]

It was federally illegal until 2013 if it's wasn't able to be detected by a metal detector: https://en.wikipedia.org/wiki/Undetectable_Firearms_Act

Some folks have tried to reintroduce it again. It's one of the few gun laws that even the NRA approved of.

[15155]

..if you didn't include a piece of ferrous metal in the frame, which most folks specifically did to comply with that legislation.

Not to mention: a magazine and all ammunition is automatically detectable (visually and magnetically.)