People shoot 45-70's a 1000yrds everyday. So long as the gun itself doesn't cause inaccuracy problems, accuracy is more a shooter problem than it is a cartridge issue.
You just have to get past the laser beam flat shooting bullet thing equaling accuracy.
My uncle and myself shoot groundhogs out to 550yrds(both have kills that far), and shoot at them even further with our 45-70's. A buffalo sized iron plate at 1000yrd wouldn't be much more difficult.