From what I've heard about Trail Boss, it's unpredictable when raising powder charges above a certail level or pressure. I've never heard anyone say it's more accurate than any other powder and what I do hear is that it makes good short range low velocity loads that will hit the cowboy close range targets with comfort.

Unique is a great low velocity powder for the 45/70 and in a rolling block or stronger rifle, you can take it up to 1300 fps with a 385 gr bullet. Somewhere between 1150 and 1300 I'd try going to 4759 or 4227 even 2400. Your already at 23K+ with your 10 gr load and only 804 fps in a 24" barrel.

For your 458 gr bullet, I would actually suggest RL7. It has about the best velocity to pressure curve for the 45/70. You can get a load with that powder that will tell you "no more" because of the pain you'll have to suffer. Either of the 4198's would be better than Trail boss for your velocity range, too.

Frank

You might also want to consider H4198 or 3031.  In both instances the pressures are under 23,000.  That is if your rifle can handle those pressures.  Trail Boss is a good SASS powder for pistols.  I would never want to use it in a rifle.