RayA - 2400 powder is at the top of my reloading data lists for the 45-70, so it's not a strange powder to use for cast bullets in that caliber.
If you are not having good accuracy with cast bullets, don't blame the micro-groove barrel - they can shoot just as accurately as a Ballard cut if they are properly firelapped. Drop on down to Veral Smith's forum for some good info and insights on firelapping your barrel, and then go to Beartooth Bullets for detailed instuctions on the materials and procedures to use. I have firelapped a number of rifle barrels and found the process to work incredibly well with my rifles. Some of the fellas mentioned bullets sized a bit large - after you have firelapped your bore you will know exactly what the diameter is, because that is what you have lapped it to and will have removed all the constrictions and leftover tooling marks in the process. Whether micro-groove or not, you will then have a barrel that shoots what you want, the way you want it to shoot (at least that's how it works for me)
I have a micro-groove barrel on a Winchester Big Bore 444 and she showed the same behavior you mentioned with your 45-70 and cast bullets. You would not believe the difference firelapping made - the choice of bullet for that rifle is now either a 300 or 330 gn cast gas check, type of game animal depending, and each bullet groups to within 2" at 100 yds.
And yes, cast bullets may be less expensive but that doesn't mean cheap. The Beartooth Bullets I use are just completely awesome. One of our other posters also shared some heavy cast.gas-checked slugs with me and that really got me into cast bullets for my big bores. Yep, it's really the way to go. HTH. Mikey.
