When a person is frothing at the mouth like that, your best argument would be to be the bigger person.
"I think that ignorance on both sides of the debate is counter-productive. I would be glad to have a fact -based and reasoned adult debate with anyone on the topic of the 2nd amendment, regardless of their views. So long as they are acting like an adult. At least when children throw a tantrum they have the excuse that they don't know any better. I am going to unfriend you now. I hope you send me a friend request once you've calmed down."
Then unfriend him.