Cost. If it works in smoothbore, it'll work in a blackpowder smoothbore. But for the price of the Brenneke slugs, can you achieve the same performance solid round ball? The Brenneke is suppose to fly farther and straighter than the round ball, hence longer range. A good musketman can place all his round balls into a piece of paper at 75 yards. Lawrence Babits discusses this in his book, "Devil of a Whipping: Battle of Cowpens." Using a replica Brown Bess, he not only achieved speed but got 5/6 shots onto the paper at that distance.