You know I've seen people say that they have to replace the plunger so that the bullet isn't defaced, but the advice is being given by people who have not been in the shooting sports for any length of time. Historically the most popular bullet in revolvers for target shooting has been the wadcutter, so named because it has a flat face and punches a nice hole in the target. There are even semi-wadcutters designed for semi-autos, for the same effect. When the Pietta plunger smushes the bullet, it looks like a semi-wadcutter, and will be no less or more accurate than a perfect cone at pistol distances. The base of a bullet effects accuracy far more than the nose.
A ball, in a 58 Remington especially, is extremely accurate at pistol distances, but at 140-150gr, is ballistically challenged. For steel plates ten feet away, they are awesome. But I have carried a BP revolver in states where my carry permit does not work for decades, and the ballistics of a 240gr conical is far superior. With the JD bullet, that unlike the original Lee bullets shaves perfectly and flies straight, there isn't a case for a roundball as a defensive round in comparison. Likewise the Walker bullet in the larger cylinder Dragoon and Walker guns.
So the other issue here I see above is lube. In a BP revolver, usually we use lube, or a lube impregnated wad, over the seated ball. This both lubricates the bore, and prevents sparks from the cylinder gap from igniting a neighboring cylinder. The Johnson and Dow bullet has no lube grooves, like a ball, so it also needs similar treatment, or to be dipped in lube as the originals were in their paper cartridges. I am doing a lengthy article and video on this and will post it when it is done. My wife just had our 7th child and I have been delayed in getting it out.