6" gives you a hell of a lot of room for error. I've held deer hearts in my hand that easily go four inches. Note, that that is the heart alone, does not include, the lungs, the arteries, or, and I note that you guys have forgotten the spine.
Even the heart from a fawn (in November) will go two and a half to three inches.
Moose hearts in general run five to six inches. They make a nice roast!
12" is kill zone for deer as far as I'm concerned, and you could extend that to 15 for moose.
i've not shot enough bear to put an oar in on that one, But I'm thinking, for black bear it's similar to deer, if a tad lower to the ground.