I suspect there are a lot of successful hunters who don't know all those things. They just point, let out their breath, hold steady and pull the trigger slowly. Not sure why they let their breath out, but the ones I know all seem to think it is a necessary part of the ritual.
None that I've ever known. And as a lifelong hunter and a lifelong member of the NRA, I've known a great many hunters. Successful hunters generally know far more about rifles than I mentioned. There's far, far more to making a difficult shot than just holding steady and pulling the trigger slowly. Successful hunters not only have to know the things I mentioned, but fifty other things, as well. No hunter is going to be successful in any way without knowing all there is to know about windage, about bullet penetration, about how different types of bullets work when they hit flesh, about mid-range trajectory, and half a dozen other things. Without solid knowledge of mid-range trajectory and windage, you won't hit your target, even if you're steady as a rock.
And you don't let all your breath out, only half of it. It's for the same reason you let out part of your breath when you try holding your breath under water. Too much air in the lungs makes it harder. Take a deep breath, let out part of it, and you can then hold your breath longer, and you'll be steadier in the process.
"Squeezing the trigger" is something I'd as soon never see again, too. You never really squeeze the trigger. And if you have any experience with a rifle, you don't even pull the trigger slowly. That's for beginners. Experience shooters generally do what's called a "trigger press." It's very fast.
Triggers are something else. They aren't one kind for every occasion. You have to know something about trigger pull, how much play there is in a trigger, the difference between a crispt and a soft trigger, a regular trigger and a set trigger, or a set trigger and a double-set trigger.
No one really holds a firearm steadily. Some are steadier than others, but no one is steady. It can't be done. Even with a rifle on a benchrest, there's just a bit of movement. Anyone firing offhand is going to be moving all over the place.
It's true enough that if you write about a television all that matters is the puicture and the sound, and true enough about cars. But it's also true that you still have to get the facts right, and when you don't know about weapons, you'll probably do stupid things with them in a story.
You know a Cadillac can't climb a sheep trail the way a Jeep can, so you wouldn't have it do this. But writers make equally stupid statements about firearms all the time.
It isn't just about guns, it's about any piece of technology the writer thinks he can bluff his way through. He can't. It's fine to say a character gets on his computer and e-mails someone. It's not fine when the writer has that computer do something computers can't do. If he does, he'll get caught.
But too many writers think all there is to hittting a target with a weapon is picking it up, aiming, and pulling the trigger. Doesn't matter how far away the target is, or what kind of weapon it is.
These thing sometimes aren't relevant to the story, but far more often than not, they are, even if it's behind the scenes.
It's fine to have a character who doesn't know anythong about firearms. But don't have that character be an expert shot, it simply does not ever work this way in real life. And do not ever have that character do something with a weapon that only an expert can do, or that can't be done at all, even by an expert.
People know how to drive a car, most people, on the other hand, are completely ignorant about firearms.
Sometimes such detail and knowledge can be irrelevant to the story, but more often than not, such details and knowledge make all the difference between a story that's believable, and one that's all bluff.
Sometimes "write what you know" really is meaningful. A character make have a reason for ignorance about a subject. The writer never does.
But the real point is, weapons are nothing at all like TVs and cars. If the character doesn't know enough to use a weapon effectively, then don't have him do so. You wouldn't have a person who has never driven a car jump in the car and be a good driver, so don't have a character who's never fired a weapon pick it up and be a good shot. And all using a TV generally involves is turning it on.
Most people have been using TVs and cars all their lives. Very few have much experience with firearms. It's seldom as simple as picking one up, pointing it, and pulling the trigger.
Odds are someone who lacks experience won't even know where the safety is, or, if it's a handgun, whether or not it even has a safety. Revolvers have no safety at all, while some semi-auto handguns have a trigger guard safety, a hammer lock safety, and a grip safety.
A rifle may also have different types and numbers of safeties, depending on whether it's a bolt action, a lever action, a pump action, or a semi-auto. Even a bolt action may have a trigger guard safety or a tang safety.
Fortunately, many writers do take the time to get these details right. It isn't difficult. If you don't know, someone else will. But when a writer doesn't take the time, and has a character do something really stupid or unrealistic with a weapon, it can make a lot of readers, and editors, put down the story and look for another.