[shabble]

A comparable but significantly lower tech solution is using a high-strength magnet integrated into a ring which manipulates some part of the trigger mechanism, unlocking it for use, eg: http://www.tarnhelm.com/magna-trigger/gun/safety/magna1.html

It's not keyed to a specific weapon, but it seems like it would help prevent the most common dangers of either a kid finding and using it accidentally, or in an actual combat situation, being disarmed and having it used against you.

The only major benefit to per-device keying would be (marginal) additional security, and increased auditability