I end up shooting a double ball load about every time I am playing with my guns. They do hit close together. I hunted for several weeks with two .530 balls in a Trade gun flinter 90 gr 3f. No nice Buck so no shot. I have a 15/16 barrel .58 cal but like it really well so won't try double with it. I have never had a ball move. But have never had a conical slide in the barrel either. Or a Deer fall in it's tracks after a shot (except head or spine). Edit I use a patch on each ball loaded one at a time.
Larry
Larry