There are no proof laws in the US, however standard liability applies. If the gunsmith was negligent in his actions and you got injured, then he (or at least his company) is liable. Here is an example: JJ fitted a scope to a merkel 470 for a customer, it doubled on him on the bench. The guy had over 5K in hospital bills and a 3” scar on his head. He tried to sue JJ, but failed because JJ proved that he (JJ) never touched the action and that the doubling was an act of god or the fault of the owner.