I will warn you that there is some dark humor in here, though.
I have shot a lot of guns, because I have family members who have fucking arsenals. I'm not an expert, but I do have experience.
Like them or not, you've probably written something involving guns. So this blog is going to be an informational thing on what writers like to forget.
Kick/recoil is a thing that I don't see a lot. Guns have kick. My cousin shot an AK once and it left a bruise on her shoulder. I shot an M4 and it really did hurt. Those semi-automatics have a lot of kick to them. It doesn't matter much if you're a big muscly action dude (or lady; just...it's the muscles that count, okay. Or whatever. Big person=the kick hurts less.).
An M-whatever (4, 16, whatever) is basically the same thing as an AK-whatever. They're assault rifles. One's American, one's Russian. The M4 I shot had a nifty little switch that switched it from automatic to semiautomatic. Which means when I switched it to automatic, I just squeezed the trigger and it fired until I took my finger off the trigger. I don't know why anyone needs these fucking things but whatever. Just. God, they're weird. You can't hunt with that. It will obliterate the poor deer. Good lord.
BACK TO KICK. Pistols, Glocks, handguns. They have lots of kick. You'll use two hands to fire these, and you will hold it rightside up. Okay? No gangsta shit. It doesn't work. And you'll fire, and if it's your first time using, say, a revolver or a Luger or whatever, your arms are going to fly upward with that kick. You also have to hold an assault rifle right or you will get a bruise. Here is an image. Do not click if you do not like to see bruises. I have heard of dislocated shoulders from some of the assault rifles, though I think that results from holding it really wrong.
Rifles really don't recoil that much. BB guns and pellet guns, too. I used to shoot a lot of BB guns when I was a little kid, and I really don't remember kick of any kind.
Things with shorter barrels will generally have more recoil. Revolvers, Glocks, Lugers, that stuff.
Nextly, let's talk about that scene in CSI or whatever where the detective is squatting over the victim and going, "Hmm, yes, they were shot with this caliber of this and blah blah blah."
I really don't see how that is a thing. Fleshy things (like, you know, us) get really nasty when shot. There's rarely a clear hole. Now, if there are bullet holes on the walls, then I guess you get a pass, because your detective could examine those.
OH! NOISE. GUNS. ARE FRIGGIN' LOUD. If you're at a shooting range, they will mandate that you wear ear protection, otherwise they're a shitty range and you can sue them for hearing loss. Because your ears will be ringing and you'll probably get a pretty bad migraine if you shoot stuff--especially with assault rifles or handguns--without hearing protection. I don't know from experience, because I've never tried it.
Shotguns can shoot lots of things at once. My dad used to shoot exclusively shotguns because he was a very bad marksman that's what he threatened to shoot people with them, but let's not talk about my psychological scars haha.
Tommy guns are weird and I don't like them. I also don't have any useful first-hand experience with them.
Err. Those are the main things, I guess. Check diagrams and stuff. Like, the barrel is the end you point at people. Actually, don't point it at people. That's bad. People don't like it. It is not nice. When you point the barrel of a gun at someone, you're saying, "I am not nice."
If you're using laser guns or phaser-type things, just disregard all of this because laser guns kind of defy physics and you can do whatever you want with them because you've already transcended realism. Make up your own rules and stick with them, though.
Please tell me if this was helpful! I might make an archery one, too! And maybe one on swords!
You know that it's actually true that some people can shoot between heartbeats? Look it up.
Speaking of looking things up, I've got some extra resources.
Like this one.
And this one! Which is just on writing fighting in general and has been super useful to me.
And this.
Hope that helped! And yes, Divided., I am fairly certain that you can tell if a revolver is loaded by looking at the front. You should be closeish, though.