Some form of energy beam, perhaps? The cynical of us might ask whether we're going to be around in 100 years time!
If you've not seen it, you might want to look at http://www.newscientist.com/article.ns?id=dn1470.
Yes i have heard of that, but I see a future in projectiles as well. Perhaps you could put together lumps of MEMS made to "track" a target at the high speeds of a standard bullet.
http://www.metalstorm.com/04_what_is_a_smart_gun.html [Broken] I found this a few months ago its interesting