I just came across this very interesting article. It made me think about how Mary shot Sherlock, or rather the shot in particular.
"A bullet that passes through the body (creating an exit wound) generally will cause less damage than one which stays in the body, because a bullet that stays in the body transfers all of its kinetic energy (and ensures maximum damage to tissue)."
Sherlock didn’t have an exit wound, the bullet stayed inside his body, meaning that it caused more damage.
Now, Mary obviously couldn’t influence the speed in which the bullet went through the room. But. Moving on.
"A bullet can shatter bone, and bone can also ensure a bullet won’t leave the body. So you’d want to stay away from areas with a lot of bone mass, such as the ribs. You would also want to stay away from nerve bundles […] And, most importantly, you’d want to keep your vital organs away from a bullet. This definitely counts your torso out and your head, too.”
Let’s see where Mary shot Sherlock.
The wound is not only in his torso but directly next to his sternum and in his ribcage. It was a shot to the lungs rather than the liver.
”[…] but since bone is dense, it absorbs more force (and damage). Bones also splinter, causing further damage as the fragments travel through the body as projectiles themselves.”
The bullet wound is close to the heart, meaning that a splinter of a crushed bone could have easily gotten into Sherlock’s heart and killed him.
So: torso, ribs (place with a lot of bone mass) and nerve bundles? Mary shot the bullet directly into a place where all these things are.
Sherlock deduced that Mary’s a part-time nurse. Supposing that’s correct and she did indeed do some nursery work under the name Mary Morstan, she would know the anatomy of the human body and thus knew exactly where to shoot Sherlock to make sure that the bullet would cause damage and wouldn’t leave his body.
In short: She knew how to kill Sherlock.
I thought Pitbull’s first name was “Featuring”.