I would think that a ring would be a better security device, the ring could contain code that when directly interfaced with the gun allows its use. The metal of the ring would come into contact with the grip during use, during contact either a specific voltage or resistance offered by the ring would allow the unlocking of the firing mechanism, release the grip and the firing mechanism locks into a safety position. One ring per gun. Maybe even a glove, or a pair of gloves that would allow the unlocking and use of the gun, gloves would allow more contact with the gun and allow for use by either hand.
The watch has one fatal flaw on both ends, batteries. The gun battery could be rechargeable and be built into the clip, for the gloves it could be as simple as completing a circuit, the gloves would have to have contact points that match the gun, they should not be standard, and could have ID's built into the gloves. When you grip the gun, the power contacts match, supplying voltage from the grip of the gun to the glove where the ID is read by the gun from 2 different contact points and then firing would be allowed. The gloves would require no battery, and different contact points at different points along the gun, so my glove can't work with your gun and vice-versa. Each pair of gloves would have both the point locations and the ID's as security devices. My glove uses (for example) contact points Z, T, H, and W, with matching contacts on the gun. Your gun uses K, L, C, and 9, so even though we have the same model gun and glove we cannot use each others weapons, If by chance the same contacts are used for 2 or more gun/glove combination's, the ID would disqualify the use of an identical glove.
This is for Law enforcement use, of course. What happens when the watch gets damaged during a chase? The glove scenario would allow the same officer backup in case of injury, his other hand. RF can be jammed, that would render the weapon useless. Having actual contact with the device and the key is the way to go. I think it would be simpler to do too.
What do you think?