1st of all, I do not have nor have I loaded for a Paradox rifle. I have loaded for a number of black & smokeless powder rifles using cast bullets, however.
From what I know about alloys and pressure caused obturation, the alloy is very important indeed. This obturation or lack of it, depends on or is also influenced
considerably on the powder used, as to it's speed of ignition, how much "blow" is given to the bullet as it is started down the tube.
Too soft an alloy, and the bullet's obturation is too much, creating extra friction in the barrel. Too little and the fit becomes more critical or the bullet cants or can thus tilt inside the bore.
Thus, a softer alloy might allow a looser fit and shoot OK, whereas a harder alloy might be more critical to fit, depending on the make of powder and/or granulation used if using black powder.