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.
"JUST A OLD DEPLORABLE THAT'S IRREDEMABLE."