Not sure what speed you get with a Round Ball but a 650gn out of a 577 Nitro for Black at about 1800 - 1900 fps hits buff hard.

I have seen them go down with one hit and not go down with multiple hits (not all good one's !!!)

Not sure on the hardness of your lead but I reckon if you angled the ball in through the ribs and into the heart lung area, it would put the animal down.

Not sure how your lead ball would go against the leg bone which is a very hard bone, hence my question above re the hardness of the lead.

I'd stay away from head shots (and the jaw) which side on will stop your bullet dead in it's tracks.