I remember reading online about bone smashing being bad for the skin(face)
Idk what the gun would do the face tbh
It is pretty legit but you got to be carful, If you punch hard and do consistent, it can fuck up you're bones that's if you punch stone walls
If you want strong knuckles, try punch the heavy bag with heavy bag gloves
I remember in Muay Thai my Ajahn (teacher) told the us to use it and then he showed his knuckles, they there big his punches would hard af.