The biggest advantage to using alloy lead for PRB is that you get to use what you already HAVE! Because alloys are hard to control, you have to weigh the cast balls, which can be tedious, and sort them by weight. For 40 caliber and smaller balls, you want to stay within 2 grains of an average for best accuracy. For .45 and larger, you can probably get away with a plus or minus 5 grain sorting. Sorting by weight is usually done with visual sorting, to set aside wrinkled balls, or those with pin holes in the sprue. Both are a good indicators that you may have an air pocket in the ball, and the ball will weigh lighter than average.
The other problem is that alloy bullets come out a different diameter than pure lead balls. So, you want to measure them, and sort by diameter. Otherwise, you can get blown patches if the wrong patch and ball are used. My personal experience with .50 cal. round balls is that the variation from pure lead to wheelweights is rarely more than .002 ", but I have seen some that are further out than that, and I can't figure out why. The only plausible theory I have heard from discussing the mystery with experienced casters is that perhaps the mold block were not held tightly together for that cast. I don't like that theory, because I would expect there to be a seam burr on the ball if that were the case. But nothing else explains why they can vary so much in diameter out of the same pot of lead.
All I know is that to be safe, you need to sort by size. I didn't the first time I cast some of these, and found at the range that some balls went into the barrel TOO easy, and some were impossible to seat in the muzzle with my short starter! I finished the match using swaged balls, and took my castings home and measure and weighed them, and sorted out. The ones- only a few- that were very light or very heavy, were thrown back into the pot for future casting. i used the remaining ones to shoot matches at 25 yds, and 50 yds, off-hand, and didn't see where I had many problems. They did strike at a slightly different POI, but I was not good enough an off-hand shooter to hold the difference in my POA.
Good luck.