So a rifle which uses an electric motor to operate the bolt is by definition, NOT a semi-auto rifle. One could imagine isolating the hammer from the rest of the guts of a rifle so that when it strikes the area around the firing pin, it closes a circuit which a microprocessor can sense on one of its pins, starts a timer which after an appropriate delay puts an electro-mechanical operating system in motion ejecting the spent cartridge and loading another. The power source to operate such a device could be built into each magazine, or in some part of the stock in such a manner that it is easily swapped. Because none of the energy of the firing cartridge goes into operating the action, it would not be a semi-auto rifle.
Even better. Use the blowback pressure to load a spring that powers the extractor, the release of which is electronically controlled or valve controlled. It doesn't take a heck of a lot of energy to power that type of system. I'm wondering if a copper round flying down the metal barrel could be used to generate the limited electricity needed to open the system, allowing the charged spring to release and eject the round and chamber another. I don't own an AR but damn if I don't want to play with this idea now...