I suspect it would be easier to make the front spin and fin-stabilise the rear than try to make a rotating core. That being said, given a known muzzle velocity I don't understand why it's not a simple timing problem.
Still easier to have temperature and pressure sensors at the weapon( they fit in a wrist watch so there should be no problem to do the same at a high tech gun) and send a rectified time input to the bullet IMHO.
