I didnt see anybody mention breathing and heart beat on long shots!
There is a reason those two items weren't brought up.
As to heart beat, you can't stop your heart, I certainly wouldn't want to, I like my heart to beat. If it isn't you have other problems then shooting.
Breathing????? One thing you can't do (or don't do naturally) is breath while your shooting. You sub-conscionably hold your breath while you pull the trigger.
The idea of taking a breath, letting half out, holding the remainder, shoot, is Hoggie.
If you're concentrating on that, you're not concentrating on your fundamentals.
If you think about it, (which you don't if you are doing things right) you shoot when the breath is at rest the longest period of time. Not when you take in a breath, certainly not after you breath and let half out,....its at the bottom of the breathing cycle.
You take a breath, you pause for an instant, then let it out, at the bottom, before you take your next breath there is a much longer pause. If you're worried about breathing, that's when you shoot.
I was shooting the Wilson matches (National Guard Championships) one year when after a string of setting rapid fire, which I shoot fairly fast, the guy on my firing point asked me if I even breathed during the 45 or so seconds it took me to fire the string.
I told him I didn't know. Couldn't tell. So the next string I tried to figure out if I was breathing, if so when,................worst rapid fire string I ever remember shooting, ruined my rapid fire match.
After that I quit worrying about it, I figure at some point I do breath or I wouldn't be here, but I'm certainly not worrying about it.
You can't breath and shoot, you can't talk and shoot (another subject) you stop both the instant the stars are lined up and you squeeze the trigger.
Forget breathing, concentrate on sight alignment, trigger control, and follow through.