If you sort the balls by weight, the alloy used is not very important. They may shoot to a slightly different point of aim, and if they are real hard, they will be larger in diameter, which you will instantly tell trying to seat the ball in the muzzle, and you may need to go to a smaller thickness patch for those harder balls. But, they should be just as accurate as pure lead balls, if sorted by weight, and by diameter. You can always have a few ingots of pure lead to add to a pot that is producing balls that are too hard.
When I was a kid, Dad had us scrounging led from backstops, from the railroad tracks, or anywhere else we could find lead. He did get some ingots of pure lead, and when preparing to make a run of bullets or balls. would take the first few castings and dump them in water to cool down quickly. Then he would put them on a scale to get an average weight . If they weighed too little ( usually more than 5 grains less than his standard) he would calculate how much pure lead he needed to add to the alloy to bring it down within his target weight. Then he would cut off enough lead off the ingot, scale it, and then add it to the mix. I was given the job of cutting off pieces of lead to meet the goal. He feed them into the pot, and waited for them to melt, and for the lead mix to come up to temperature for casting. After a few bullets or balls were cast, he would again cool a couple down, weight them and check them for diameter, using his vernier caliper, before proceeding on, if his calculations achieved the desired mix. We enjoyed the challenge of making " Free " scrap lead work for us.