I had a .45-70 many years ago, the one with a heavy 1 & 1/8" octagon barrel. With the tang sight and 400 gr Speers it would do about 1 MOA at 100 yards. I never did try lead bullets & BP in it, but it seemed to shoot fine with jacketed.
With the hammer down I'd check to see how much play is in the breechblock. Also check to see that the sear is not worn. Cross will likely have additional insight.
