If you're making such an app, you'd be filtering the content for obscenities anyway. I mean, Unicode already includes a swastika character [1]. Wouldn't you just add the gun emoji to your list of censored words and emojis?
Per your link, the swastika is a religious symbol for a large number of people for a long time. It's strange to suggest that it is an obscenity just because a small number of people used it to mean something bad.
[1] https://en.wikipedia.org/wiki/Swastika#Written_characters