Well, personally, I'm only 21 so I don't have THAT many years of dealing with the fairer sex (I only started noticing girls when I was like 10-11 or so), but so far here's what I've learned. They seem to value honesty. So my advice is to be open and upfront about it. Think about it this way, you are dating someone because you like them, and potentially, that person may very well end up being the person you might spend the rest of your life with, so my theory is to be open and honest about things from the beginning, that way if it's not going to work out, it happens sooner rather than later before anyone gets too attached.
As far as girls and guns go, surprisingly, I've noticed that many females do in fact have an interest in guns, or can easily be "converted" to at least tolerating guns if you present them with logical facts in a clear, intelligent, non patronizing manner. A few examples. A few years ago, before I could even legally buy my own guns, or even had any guns, I was already very pro 2nd. I was about 16 and I my cousin who was 17 at the time was a card carrying NYU liberal. She still is, but thanks to talking to me alot, she sees why people own guns, and generally doesn't have a problem with them at all.
Fast forward to about 2 years ago. My ex fiancee (long story) was surprisingly VERY pro gun, and loved shooting. I was shy about talking to her about firearms at first, but after I learned that she was as big of a gunnie as I was, things went very well, at least in that area of our relationship...
And today, my good friend Kim (who is 20) is showing a very healthy interest in guns. The first time I took her out, was to a shooting range. Both her, AND her friend had a blast that day(pun intended), and she loves going shooting with me. I even talked her into getting a handgun of her own once she turns 21, and yesterday we were talking and one of the first things she asks me is "Hey Joe, when are we going shooting again?"
Even my own mom, who was VERY anti gun, now owns her own gun, and loves to go out shooting.
Yep. Truth be told, I've never really run into anyone who is a true anti and is closed minded. All of my friends have shown an interest in shooting when I bring it up, and this to me is encouraging, as most of my friends are in the 17 to 23 year old range (again, I'm just 21), and people of that age are tomorrow's voters/activists.
All one has to do is be open about it, present it as a normal, everyday thing (which it is), and not make a big deal out of it. If you present yourself as a sane, level headed, polite individual (which I am sure you are, just like most of THR members tend to be), then she shouldn't have a problem knowing that you are carrying, and will probably appreciate the fact that you are responsible enough to take charge of your own security, and the security of your loved ones. That's pretty admirable if you ask me.