system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How come in the 1700s a single musket ball to the torso could kill a man, but now it may take 2 or 3 shots from a modern firearm to do the same?
gpt
This isn't quite true: a modern high-calibre sniper rifle or a .45 magnum will kill you quick with a body shot. But if you're talking smoothbore larger muskets compared to smaller sidearms, the answer is calibre and shock.

A single musket or minie ball is very large and very heavy compared to a standard sidearm bullet, and although it typically moves slower it has a lot more force behind it. When it hits, it slams into a person's body rather than punching through rapidly like a faster revolver bullet, transferring all of its shock into the surrounding tissues and crushing them like a hit with a sledgehammer. That's enough to almost instantly kill someone when it hits them in the abdomen, at minimum knocking them out of commission until their bodies dies from the trauma. 

There's other factors such as infection and modern medical treatment, but they're more about surviving for days after the person is shot, not about whether the shot immediately stops them in their tracks.