in order to transplant a head you wolud need to be able to attach thousands of nerves to each other, you would need to do it by remote using miniature robots for the surgery, it wolud be extremely difficult and i cant think of any case where it wolud be necessary to do, the risk is such that it wolud only be done in a life or death situation, but the real problem is finding a brain dead donor with working organs, accidents usually damage the organs causing the brain to die, and anything like a gunshot to the head would be investigated for days and the body would not be suitable for a transplant anymore
