To know the brand and caliber of your B/P rifle would help. Knowing that information alone would give many of us a chance to offer helpful suggestions. Your 6" Colt would be sufficient at close range with a 158 gr. average store bought bullet. But, I suspect there wouldn't be much room for error. As your aim should be >Spot On. Mullies are indeed a BIG animal.
"Do not follow where the path leads, rather go where there is no path and leave a trail for others to follow."