Drinking is no excuse for waving a gun around. I'm sure she is going to call tonight, say "I don't remember" and then she is going to expect you to say "everything is fine, no one got hurt".
As a recovered addict, I strongly believe that addicts need to feel the consequences of their actions. If this is just brushed under the rug then she won't feel the negative consequences of her drinking, and the cycle of drinking will continue. If this was a friend that came into the house and waved a gun at two people, how would you react? Sometimes, taking out the fact that she is your daughter, and reacting to the situation as you would if it is someone else, helps not to enable.
It sounds like her drinking is escalating and getting dangerous. Please make sure that from now on she does not have access to a gun when she is in your home. She is a grown woman, she knows exactly what she is doing, and there is no excuse for threatening people's lives, whether it was a joke or not. ( I would like to point out that if she doesn't remember it, she can't say it was a joke.)