My Daughter is 2 years, six months.
My approach with her on everything, not just guns, is to reduce whatever we're talking about to sound-bite format, and use Q&A to quiz her about what she's learned.
Her first introduction was a Nerf pistol that single-shoots a golf-ball sized sphere of foam.
I showed her how it works when she was about 20 months. She couldn't pull the trigger like me so we figured out how to two-hand it and put both index fingers on it.
She had to be very deliberate with it and put some muscle into it. It makes a pretty good POP! when it goes off, so she naturally was a little hesitant, and naturally wanted to make sure it was shooting in the right direction.
She eventually pointed it haphazardly, and my wife and I, without yelling, said "WHOA WHOA WHOA! No pointing at people!" She got the message pretty quickly.
Eventually the cat walked by while she was shooting it, and we called out "WHOA" again. (We were going with what we usually say to her... at that point "cease fire" would have been meaningless.) We explained that there's no pointing at the cat, either.
So without thinking about it, we were teaching her some of the fundamentals without using the actual fundamentals.
Sometimes you need to stop shooting --- WHOA!
Make sure it's pointed in a safe direction --- not at people or the cat. Also, the safe direction becomes unsafe when someone walks out in front.
I'm going to work on trigger discipline when she's older, and also being sure of what's behind the target. We're working up to that...
The current quiz is which guns she's allowed to play with. We started with green guns and red guns (the color of the toy guns we have). I always thought this was a stupid thing for toy manufacturers to do, but now I appreciate it.
We also started with how she's NOT allowed to play with black guns or grey guns (what she's likely to see in the house). For a two-year-old who speaks and knows colors, that's pretty easy - two good colors and two bad colors.
It was so effective that when we went to her brother's house a couple weeks ago, they had a yellow nerf gun... and she wouldn't play with it, because it wasn't red or green. Also, I was leafing through a magazine last night and showed her a couple mostly-furniture long arms (brown), and she said she's not allowed to play with those, either.
My wife doesn't like me explaining to her that she'll get to use them some day: she wants success to stay success. But last night, I figured out how to work that in: the kitchen knives. She knows they're sharp, she knows they're dangerous, and she knows she's not supposed to touch them. But she also sees Momma and Poppa using them every day. So I said that guns are like that.
Of course, there's a lot of other stuff that went into this. She's been signing since 8 months, and that's supposed to help with speaking a lot. We spend a lot of time just interacting with her, and know exactly what she's capable of.
We made a lot of headway on the guns, but there are still things that concern me, like not getting through to her about not putting her fingers down her baby sister's throat. And if we're not getting through on that, can I be sure of the gun progress?
In any case, I keep telling myself that Dr. Lott's statistics show that the babies-shooting-babies thing is a myth, and I keep doing what I can.