I'll admit it, I've made snap decisions about people based on their gun handling. However, I think those snap decisions are safe ones. For example: if you're out at the range and you're sweeping everyone nearby with the muzzle of your gun, I'm going to assume that you don't understand the responsibility of safe gun handling. I'm going to immediately assume that A) you're a self-centered, lazy asshole, B) your a freaking idiot who couldn't be bothered to learn good gun handling (which is an awful lot like option A), or C) no one ever taught you any gun manners.
Sadly, my assumptions always come in that order: A, B, and then C. Those assumptions are generally based on experience. At one point in time, I would take the time to explain the range rules and tell the person how dangerous their actions were. With *one* exception (which lead to assumption C), the offenders were of the A or B category. I've found that pointing out the error of their ways, no matter how nicely I tried, was just wasted breath. Now, I just pack up and leave. It's not worth potentially getting shot because of a negligent discharge, nor is it worth getting into a verbal altercation.
There is basic gun etiquette that I expect all shooters to follow and I'll judge you on it. Just being honest folks.
The first thing: if you want to show me your gun, please point it in a safe direction and "make it safe", which means drop the magazine and clear the chamber, all the while keeping your finger OFF the trigger. Show me that the gun is clear before handing it over and hand it over without pointing the muzzle at me.
Understand that I am going to physically clear the gun myself. Don't be offended by it. It's just good gun handling etiquette. I promise that I won't point the gun at you while I'm clearing it either, and I'll be sure to keep my finger off the trigger.
Few things make me more nervous than someone wanting to show me their gun and just handing it over, or worse, leaving in its case and handing it over. I don't care if you tell me it's loaded or unloaded. I want the gun cleared before I get it, and I want to see you clear it.
When handling your gun, keep it pointed in a safe direction. Namely, away from me! I feel much better when someone turns away, keeping their body between me and their gun, to drop the magazine and clear the chamber. Don't worry, I'll peek over your shoulder (or under your arm) while you lock the slide back so that I can see the empty chamber for myself. And then I'll repeat the process on my own once I've got the gun in my hands.
When out at the range, PAY ATTENTION! You're not the only one out there. Never break the 180 degree rule. The muzzle of your gun should never break the firing line in either direction. In fact, the muzzle of your gun should be pointed down range AT ALL TIMES unless it's in your holster. Even when setting the gun down on the bench, keep the muzzle pointed down range. When loading and unloading your gun, keep the muzzle of the gun pointed down range. When clearing a jam, keep the muzzle of the gun pointed down range. When talking to your shooting buddy standing next to you, keep the muzzle of the gun pointed down range. See the pattern here?
Finger off the trigger, buddy. That trigger's not a convenient place to rest your tired booger hook. If your finger is so tired that you need to rest it, put the gun down (with the muzzle pointing down range). The only time it should be on the trigger is when you're fixing to press it. I can't tell you how many times I've seen people at the range with their gun dangling from their hands by their sides and their fingers on the trigger while they talk to their shooting buddies.
I've seen some life-long shooters who have gotten complacent with their gun handling, because they've done the same thing a hundred times before without a bad outcome, so why would this time be any different? Just because they've gotten lucky that nothing bad has happened in the past is no guarantee that it won't happen in the future. Gun complacency is the reason why people get shot with "unloaded" guns. Those people have mishandled their guns so frequently that they've forgotten how dangerous they can be until they get a painful reminder. Kinda like this guy...
Now, on the other hand, someone who follows the basic gun etiquette and handles their gun as though they've got a brain in their head is someone I'll associate with. See, to me, good gun etiquette means that you give a damn about yourself and those around you; you don't needlessly or recklessly put yourself or others in danger. And I appreciate that.