[Accujack]

You can, but it's not as simple as it sounds. Since the timing of the explosives is critical to achieving a nuclear explosion, you can't really do the timing all in software. You need a specialized network of switches that splits the detonation signal several times so that all the explosives go off at exactly the same time. Even the wires have to be cut to the same length with a tiny margin of error.

You can replace the timing and electronics mechanism prior to that network, but probably any sort of tamper resistant mechanism for a weapon will remove part of that network of switches if removed from the device. The rest of the controls are just too easy to replace to be effective at keeping someone from a roll your own type solution.