If there is a conversation at work about guns, I usually get included by virtue of rank and the fact that I don't spout off nonsense every time I open my mouth. There are a large number of shooters. Happens sometimes in the military... (though there is a single distinctly anti-gun liberal in the bunch, good thing we are all engineering types).
If I am in public, and I hear something that is wrong or that I feel needs correcting, I simply look over in their direction. Most of the time, I get noticed, and if I do, I state a fact, and then tell them they should research it. Usually a quick Google search on the relevant topic finds supporting or contesting evidence. For instance, on Wednesday, I was at physical therapy and there was a group of ladies speaking about having a gun in the back seat. I simply looked over and smiled and gave a little shake of my head and informed them that they should look it up, but I was pretty sure it is legal here locked and loaded in the glove compartment.
I try not to overtly listen in on conversations, but if you are talking loud enough in a public place, your conversation is no longer private. Most of the time, I don't want to hear it. And most of the time, I don't bother arguing with a stranger. If they are in doubt, I try to point them to a good source of information. If I hear something dangerous, like shooting the wrong calibers out of the wrong gun, I will say something.
Most amusing is when you see a salesman giving someone a line of bull at a gun store if you just smile at them and raise your eyebrows in an "Oh, really?" they will often start reeling in the BS.
But to be totally honest, it seems that 95% of our population has forgotten that other people don't really want to hear what they are saying all the time. The Borg earpieces get me more than anything. The worst part about having to listen to a conversation that you didn't want to is to only be able to hear half of it.
The only time that I recall ever straight butting into a conversation was when I overheard a salesman tell someone looking at bullets and reloading equipment and told them that the grain size of bullets was the powder weight... I wasn't sure that they could put that much in a pistol cartridge, but I felt it would have been unethical for me to let that pass. The salesman then started to argue condescendingly with me, telling me that I didn't know what I was talking about and I had to get the *gasp* reloading manual off the shelf and open it up to make her stop. She stormed off in a huff, and the other customer shook my hand and walked out with the manual, saying that he guessed he needed to do some more research before he started reloading.