There are some people who you can debate with and show that they’re wrong. You can present facts and evidence and they’ll understand their preconceived notions are incorrect. They understand they don’t have all the answers and are willing to entertain new bits of data that go against what they previously believed.
These people, however, are rarely the die-hard anti-gunners.
For the die-hard, no evidence will ever be enough. Take this bit from an article on how churches are using armed parishioners to help provide security.