I loaded up some 405 grain hard cast lead bullets from National Bullet Company for the 45-70 and went out and tried them out yesterday.
I can't get any kind of accuracy out of these bullets at all.
The rifle is an H&R Ultra with a 22" 45-70 barrel from the accessory program. The rifling is micro groove. The load is a 405 grain RNFP hard cast with one lube groove. I used H4198 powder, and Federal LR primers. They were loaded to just kiss the lands, and that put me right at the top of the crimp groove. I applied a medium crimp to them with the Hornady New Dimension dies that I use.
After every five shot group, I punched the tube with a patch soaked with Hoppes #9. I noticed that the patch was coming out the end of the barrel covered with "slivers" of lead. Once I got these slivers out, and punched it again, I got mostly powder fouling on the patch.
Is it possible that because of the micro groove rifling, I need to size bullets for this rifle a thou or two smaller? The bullets mic'ed at .458" so I figured that they were ok to shoot in this gun.
On a positve note, the 265 grain hard cast bullets and the 300 grain hard cast bullets shot great in my 44MAG Super Redhawk. It really liked the 300 grainers.