I have a buddy who deer hunts in the Apalachicoa River swamps near Blountstown, Florida. I was a bit upset the first time we went out hunting, as he was using a .22 Mag!
Granted that most of the deer are rather smallish, but he was regularly successful with heart shots. Damfino. Not my style...
I've used the 85-grain Sierr HPBT exclusively in my .243 and have never lost a deer. I dunno, maybe killed 20 or so with it...If they're looking at you, shoot the white spot, and they fall dead. I also agree about hitting the lungs near the heart, rather than the heart itself.
I won't use the .243 on West Texas mule deer. Just not enough gun, for a body shot.
I shot one muley with a .30-'06 at maybe 30 yards, right through the neck. I was truly astounded that the bullet did not exit! On CenTex whitetail, there was always an exit hole of significant size...
Count me in with those who believe bullet selection and shot placement are the prime factors.