The origin of the 21-gun salute is in the frigates sailing the seas in the 16th, 17th and 18th centuries. Each of these war frigates had 21 guns. When coming into a friendly port, the guns would all be fired as a way of showing the peaceful intent of the visitors by making them totally vulnerable.
Allies or surrendering enemies hold up their hands as a symbol of being unarmed and thus vulnerable.
I believe that Abraham and God did a show-and-tell these in the story of the binding of Isaac. Four times previously, God had told Abraham that He would make a great nation of his seed, and specifically of Isaac. What better way to signal the end of human sacrifice and purposely canceling it forever? Mt. Moriah, the three-day trip, was the city center at the time the story was heard. Abraham had to be in on the plan. Was Sarah? Was Isaac? We know that Sarah didn’t die in Beer Sheva and whatever he know or intuited, Isaac kept very quiet and under the radar thereafter.