A deer's front shoulder covers about 1/2 of the heart and when selecting where to shoot, I aim for the 2:00 position of the front leg - placing the crosshairs straight up the rear portion of the front leg and squeezing the trigger when they are about 1" below the top of the shoulder.
Aim for the heart. If you're a little off, you still hit the lungs, anyway. Also, if the deer jumps the string, better chance of a hit if you aim for the heart. (imo)
Dave has a excellent point, however, I always aim for lungs but aim low in case the deer ducks at the release, actually they jump but they will be crouching when the arrow arrives unless its a long shot or they catch movement of your draw.
What does "gauge" mean anyway? As used here gauge means the number of round lead balls the same diameter as the inside of the barrel (aka the bore) that it would take to weigh one pound. It takes 12 lead balls the same diameter as a 12 gauge barrel to weigh a pound. The smaller 20 gauge would require 20 balls of that barrel diameter. The larger bore 12 gauge would require 12 lead balls of the bore diameter to weigh one pound.
In general the number of pellets in a shotgun shell...