If the nuns really give a shit about gun violence they would go to cities like Chicago and explain to the inhabitants the importance of a two parent family where at least one of the parents has a job and the other parent raises the children.
Then instead of a bunch of fatherless gangbangers running around shooting each other you'd have law abiding gun owners who use their guns responsibly.
C'mon man! Even nuns aren't that
It's much easier to try to bully honest, law-abiding citizens than to try to take firearms from gangbangers.