I've been fighting the exact same problem with a custom hollow base slug. They cast perfect, but don't fly straight no matter what. I'm starting to think having that much weight forward is detrimental for a balanced slug. I've seen it on taufledermaus where a hollow base slug that flies like a lazer from a smooth bore, is cork screwing from the rifled barrel. I'd bet mine would shoot good as a plain base, but that would bring them to almost 950 grains.