we had a guy at our studio seriously into this. you have to train a lot: hit bags of rice, then beans, then sand, then steel shot. eventually yes it tends to cripple your hands though that can be somewhat mitigated by rigorous mobility exercises and basically PT. still do not advise.
note that almost any fighters knuckles will be more solid as the lattice structure of the bone gradually fills in. that doesn’t cause as many mobility issues, it’s when you get new growth on the outside that problems start.
This reminds me of a clip I saw of an older martial arts practitioner, like heavy-duty, far east, everything was translated. He had spent decades punching harder and harder things until he had to hit opponents at a small fraction of his strength, as his forearms on up were mineralized clubs on the inside. A full strike could kill.
I watched closely and his hands didn't seem to be much use for anything else.
note that almost any fighters knuckles will be more solid as the lattice structure of the bone gradually fills in. that doesn’t cause as many mobility issues, it’s when you get new growth on the outside that problems start.