You never mentioned what type of ammo you are using... factory smokeless or handloaded? You might try a box GOEX Black Dawg or Ultramax Cowboy smokeless. Good ammo is critical to making any rifle shoot well.
As a noob to "big caliber rifles" myself, I had a learning curve to overcome.
Things are very different when going from a loaded down 45lc to a big bad 45-70 or 45-90. (the later in my case)
I HIGHLY RECOMMEND STEVE GARBE's PRIMER!!!
(http://www.blackpowderspg.com/ART/primer1.gif)
Slug your bore first, I know Gander Mountain and others will do this for you if you don't want to bother. Meister makes a REALLY nice slug kit. (.449" to .462")