My great uncle worked for the Reagan administration and when dealling with liberals he used to say you can not be a child and not ave the heart of the liberal, and you can not be an adult and not have the mind of a conservative, Because while your child like heart wants to welcome everyone and give everyone what they need. Your brain reminds you that first someone has to pay this this as nothing is free and Second giving someone something because they want it will only make them want more.
I think that is from Winston Churchill. Something like "If you are not liberal in your youth you have no heart, if you are not a conservative at 40 you have no brain."
Personally I've ran a little counter to that. I have been much more conservative in my youth but as I've gotten older, the more it looks to me like people should be left the hell alone. Gay, Muslim, Christian, Atheist, black, Latino, etc. So long as you are not harming anyone's else, you should be able to do whatever you please.
To the original topic at hand, a good tactic to employ is to appeal to progressives natural inclination towards the weak and down trodden. Ask why they want to leave physically inferior women and the elderly powerless against much more powerful criminals that can over power and victimize them. Firearms give the weakest among us a way to level the playing field against criminals that would have their way otherwise.