Since "smart guns" do border on science fiction, its fair to look at some in SF for ideas and examples.
First, in science
fiction, they always seem to work.
One interesting idea was used in the Lost in Space movie.
In it, the guns were "voice activated". And the guns could be told who was authorized to use them. One part of the plot involves DR Smith (who wasn't on the list) convincing young Will to tell his gun to authorize "any" user, and so, the bad guy gets his hands on a gun that will work for him...
So, here is the idea of voice print recognition, along with the gun being able to store a list of authorized users. This is head and shoulders above the fingerprint ID system in several ways. First, the multiple users ability, and second, not having to grip the gun in just the right way, being able to use gloves, etc., and its better than a watch or some other separate item that could be lost, damaged, or simply malfunction.
Of course, there are drawbacks to that as well. So it would only be good sense to include a silent means of id for the gun, as well.
Current and near future tech isn't reliable enough, but in 100 years? who knows?