I think the NRA is important and I am a member but I would agree that it will do nothing to convince a liberal.
What I find often helps people open up is the realization that gun owners aren't necessarily their cliche good ol' boy type (like me: artsy dude, etc.). I've had a number of discussions and I fare best by keeping them about ideology (rather then going down the "what type of guns are ok" rabbit hole) and why I think as someone who was born and raised in a dictatorship the 2nd Amendment is not just important but the right of all rights. I then bring it around to personal responsibility and sprinkle a little of this
in there which so far nobody dared to disagree with and usually that allows me to occupy the moral high ground pretty quickly.