These could be used for a form of gun control (gun barrels and shells), reducing cycle/vehicle theft and other items.
When extruding metal for using these applications, use metals which bond well, but do not mix, and extrude two tones in a bar code as part of the section. This will mean it cannot be filed off - as it is an integral part of the structure of said item. It may simply be a case of applying a permanent tint/pigment to sections of the metal as it is extruded.
Bar code scanners are fairly easily adapted to this, and ubiquitous enough for police and forensics to have them. However, the machines to form it are non-trivial, so there is still great scope in thinking through methods to apply it.
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