It's only illegal when it happens to violate the definition of a machine gun in federal law (multiple rounds fired per trigger pull); there's no specific law about electronic triggers. The ATF has specified that electronic firing is acceptable as long as a single physical action by a human results in a single round being fired from a single barrel. A drone with a gun that fires a shot in response to a button press from a wireless controller would be legal (ignoring any FAA restrictions on guns mixed with aircraft). Electronically "storing" human trigger pulls for later use is not allowed, so it would never be able to fire on its own.