I think you'll find more accuracy in the .44 than the youngster can get out of it. I also think you'll find that 240 to 270 grain bullets will be more than adequate for deer and black bear. At the ranges you will likely be shooting them, well under 200 yards, I'd guess. And range has a bearing on stabilizing the heavier bullets too. A bullet becomes unstable when its rotational speed drops below a certain point, so again, out to a hundred yards and a bit more, it will probably shoot the 300 grain bullets as well as the 240s. External ballistics gets quite complex, the simple rules you have read here and above are correct for conditions sometimes but not all times. Each time there's a change in the length (weight) of your bullet, or twist of the rifling, or velocity, the answer changes. Don't worry about it; just get a load with a 240 grain bullet that will stay in 4 inches at 150 yards and forget everything else.