Check the crown for fouling or damage. Also the 405 is too short for that rifle. The lyman postel 535 should work better. I have 2 c-sharps in 45-70 and I gave up on the 405 yrs ago. Another problem would be the lube, should be useing spg or lyman black powder gold, don't use any petroleum products.
I had two Pedersoli Sharps rifles and they both shot 405g bullets great. With very low velocity loads they would tumble. Bumping up the charge made a huge difference in accuracy. I've had groups under an inch with mine on occasion. Typically they would be just over an inch off bags from a bench.