One approach is to use a Dremel cutting wheel to slightly weaken the brass, so that it can be pulled back in strips or petals.

Might want to think twice about taking a dremel cutting wheel to a loaded cartridge, especially one loaded with black powder. Brass heats quite quickly and black powder doesn't take much heat to ignite and that could really ruin someone's day! A bullet puller like RCBS makes, as suggested earlier, is the safest way to break down a loaded round. But to each their own...WBLDon