You haven't mentioned the type of bullet lubricant you are using. If you aren't using one of the LBT lubes, expect leading. I spent 6 months full time and a small life savings developing it, and there is nothing on the market that I'm aware of that comes close to its performance. In my experiance all the others are limited to much lower velocities and tend to operate with a bit of leading always present.
I've never worked with a 45-70, 90, or 110, but have burned a lot of powder in a 458 Win, and expect it's powder capacity to be similar to or a bit more than the 45-90. -- Powder capicity and ease of ignition are the issues in getting good powder burn with reduced loads in large volume cases. In other words, the powder must light up easy and consistently to obtain accuracy. 4198 is hard to beat for easy ignition, and gave me some of my best light loads in the 458. Most 45-70 shooters swear by it also, for target loads. I've had my best luck with the faster ball powders intended for 222-223 size cartridges. The ball powders burn cooler than stick powders, which minimizes leading. I've experimented with getting maximum power possible with cast bullets using 4198, in several cartridges, and found that it could be worked at higher pressure than any other stick powder I had on hand, and I had most that were available then, in the late 80's.
