Bullets will work, but they are the "quick" way of getting an elk load. Round balls of that size are all that is needed and actually kill
better (higher vel. and flatter trajectory) than the slower moving and poorly stabilized slugs.
The slugs Mike showed at 310gr., are likely the best bet as far as slugs go, in any slow-twist round ball barrel, ie: 48" and more. 500gr. or
450gr. will not retain stability after impact, thus straight line penetration is a hope, not a promise. Turning 50 to 90 degrees at impact you
do not want. A perfectly center of the lungs impact that turns and pens the guts instead is on your head, if you use slugs in a round ball twist.
Don't do it. Learn to use round balls with appropriate loads, is my suggestion/solution.