Sharp, I haven't had any experience with elk using a ML nor with the 58 cal. Damn that's a big bullet. My experience with round balls have been with 50 and 54 cal. (mostly 50s) Up until a few years ago here in PA you could only use round balls. They can be very accurate provided you use the right patch thickness and powder volume for your rifle.
Another thing, don't expect to see much of a blood trail from round balls. Several deer that I have hit good (hear/lung area) did not bleed at all externally except a few tiny drops but were recovered by careful tracking. For some reason conicals seem to make an animal leave a better wound channel.