Something along the lines of RFID? I've heard them described as "radio controlled" before too.

Since it's in the chamber protecting it from external sources of interference / attack (e.g. programmed to explode at the end of the muzzle) should be easy to protect against physically. I guess you could do it with some mechanically retractable pins or something, but electrical contacts and the inside of a bullet chamber don't seem like the sort of thing that should go together to me. You'd need to clean them constantly.

Presumably they got their security right; no recommit after firing, for instance.

If not, I can easily imagine a "bullet jammer" gizmo that broadcasts a retargeting command. "Sorry, little grenade, you have another thousand meters to go...."