I shoot flint / cap lock guns, no experience with modern inlines/break opens. But I'll take a few educated guesses. Modern in-lines are rifled pretty fast for sabot use. 1-66 twist is about right for a round ball. 1-48 or so works well with heavy lead conicals. I think it is a combination of overly fast twist, and the fact that a lot of in-lines are rifled shallow. Heavy, bevel base conicals may be skipping the rifling, with a little gas cutting to boot. You could try a conical sized right to your bore, and cast harder then pure lead, but that would be hard to load. I would look at a conical like the Hornady Great Plains Bullet, with a hollow base that will expand for a good gas / rifling seal. Load it with about 80/90 grains BP. It might work for you. Its the only conical I hunt with in my 50's and 54's.
Larry