That looks like a .22 bullet to me, cheap one, no copper coating just lead, mic it and I bet it is very close.
I cut a rage broad head out of a buck's neck roast 4 years ago, it was encased in a grisel like tissue, very tough to cut with a knife. This shocked me because, well the guys who use Rage heads claim nothing lives once they get hit with one, you can see how I was confused right? I have butchered all my own deer since I came home from the service over 30 years ago and have found a few bullet wounds, broken bones, and some anomalies I just couldn't identify too. Tough is what a whitetail is, tough as nails.
As far as the bullet in the heart, that buck must have been a cool customer, a human heart surgeon told me once that most hearts when touched by anything foreign go out of rhythm as if it were a heart attack almost instantly, My dad gut shot a buck through the ham with his 35 Remington, and it dropped and kicked like crazy and died. I gutted that buck and the bullet had gone through the guts and liver, can't remember what lungs looked like but it stopped at the pericardium, the sack that encases the heart. The heart was even bruised, but the pericardium wasn't broken. I figure he gave the buck a death punch with that fat slow bullet.
Sadly most of my life has been wasted, but the rest I spent hunting, fishing, and trapping...