Okay, just reading your post and responding- I don't really have the time to read 7 pages of responses! Anyway, I'm speaking from personal experience here. I have grown up in a household with guns, and my children have been in households that have guns. They are everywhere even if you don't realize it.
So the question is about playing around with toy guns and shooting people? It sounds like there are two issues here: One is that you don't like the play shooting (and especially being the target), and the other is that you are worried that the play shooting might lead to a real shooting in the case that he actually gets a hold of a gun?
First, I cannot underscore the importance of teaching gun safety. Most likely, you have been in several houses with guns and haven't even known about it. Many gun owners choose not to disclose that they own guns, and some gun owners may not be very cautious about gun storage. Chances are, your ?grandfather? (or was it your DS's grandfather?) is a great person to teach this. This is how we've dealt with it in our household. We, of course, give the whole schpeel about how if someone is hit by a bullet it can maim or kill them. My oldest has seen some violent media (not a part of the gun safety training), so he gets the point, and we're quick to point out that in real life the person shot does not just get up and walk away. We've talked about it at length, that if a trigger is pulled, the bullet has the capacity to kill. THere are some basic safety rules, I'm sure you can find a comprehensive list of these online.
The first rule that applies to every child is:
If you see a gun, and you think it might be real, DO NOT TOUCH IT. Run and get an adult. Even if it might be a toy gun but you're not sure.
The general rules that apply to young and old:
Treat each and every gun as if it's loaded, even if you think it isn't.
Do not point a gun at something unless you intend to shoot it (clarification here is needed for a 3 yo- maybe simply "never point a gun at people or animals")
Always keep your finger off the trigger until you are pointing the gun at what you want to shoot (this is called trigger control)
Always point the gun at the floor when you are looking at it (muzzle control)
If those rules were always followed, no one would ever get unintentionally shot. You can drop a gun and it won't go off. The only thing that will make a gun go off is if you pull the trigger while it's loaded and "cocked" (there's a round in the chamber) with the safety off.
That basic education, given as occasional reminders (in the same way that you might randomly quiz him on what he would do if he got lost) is only part of what I consider to be a true safety course on firearms. The second part isn't for the squeamish, but I consider it to be very important as well (and yes, I think 3 is an okay age for this, but you're his mom so you know best).
We all know that kids like to explore what intrigues them. And if something is completely prohibited, then it's even more interesting. That's why I consider it to be almost as important to let the little one actually handle a gun IF HE'S INTERESTED. Have someone who is completely familiar with the gun unload it, and double check that it's unloaded (no round in the chamber or magazine). Do this in front of the kid while talking through the gun safety rules again. Then carefully hand the gun to the child, reminding him of the rules (tell an adult IMMEDIATELY if you see a gun; never touch a gun unless I'm present; when handling a gun, keep the muzzle down and finger off the trigger; treat every gun as if it's loaded). Let him look at the gun, making sure that he's following the rules while he does so. Then, let him point the gun at an inanimate object and attempt to pull the trigger. Put the gun away carefully after he's done exploring, and remind him of the rules again. Then let him know that anytime he wants to look at the gun, you will try to say yes, but he ALWAYS needs to ask and you must be present. Keep the gun under lock and key. Whenever he wants to look at the gun, try to say "yes" and then repeat the rules as he's looking at it.
This little exercise is beneficial in a few ways.
It helps to satisfy his curiosity about what a real gun looks and feels like.
It brings the issue out into the open, facilitating further discussion.
Having him actually pull the trigger allows you to see when he's actually physically capable of doing so (though keep in mind that guns vary in this respect, but your average 3 yo isn't going to have the strength or hand size to pull the trigger on your average gun).
Actually handling the gun and practicing the safety rules will help to cement them in his mind.
As far as playing with toy guns, I view this as inevitable. I don't really think that refusing to let him have a toy gun is going to help keep him from being hurt by a real one (knowing the rules and keeping the lines of communication open are much more likely to prevent injury or death). But if being "shot" or seeing him point "guns" at other people or animals makes you feel icky, maybe you could tell him so. Maybe you could express that you know he's just playing, and it's just a toy, but when he does that you can't help but think of what real guns do to real people. I've read the "Playful Parenting" take on playing with toy guns (making it a kissing gun), and while I think that has its merits, it left an icky feeling my stomach as well because I consider this to be a very sober topic. So I would try to be honest with him about it.
There was more than one occasion I was exposed to peers holding and 'playing' with their parents' guns and on those occasions I knew to leave the room at once.
My dad had us practice shooting with an air gun (bb gun) when we were still in elementary school. I'm so thankful as I look back for his straightforward approach and I think its the only approach that'll prepare your children for inevitable situations.
Also, I have no issue with DS playing guns as long as he's "shooting" people that are playing along. I remember cap guns and small guns and playing games with all the neighborhood kids growing up and had a blast (and I was otherwise a super gentle and shy gal!)