450- Is the slugmaster rifled? If so, round balls probably won't shoot as well in it as a smoothbore. The twist is way too fast for them at any worthwhile velocity. If you slowed them down to about 800fps, they'd probably shoot accurately, but be of no use.
: A friend of mine and I are going to rifle a couple 12 bore barrels this coming summer with something around 80" to 100". These very much slower twists will deliver very good accuracy with round balls. The 35" twist (or therebouts) in the rifled 12 bores today, is about right for a very short sub-12 bore bullet - like a 450gr. .50 cal. BP rifle slug. The full sized paradox bullet would be better yet in about a 50" to 80" twist.