I wish I'd had someone to give me the sort of info you have just given Steve, when I first started casting my own, I found out the hard way. But now I do it almost to the word of what Steve has described and have been problem free ever since.
Using scrap lead is fine (I still use it now), but getting a 9Kg roll of lead flashing from your local builders merchants adds a bit of luxury, as there's no cleaning etc involved, doesn't make the slightest difference to the balls though

One thing I did try, when I started competitions, was to weigh and check the size of each ball and put them into groups of similar weights etc and the best ones I rolled between a sheet of toughened glass and a faced block of steel, resulting in balls that had no sprue marks and were as round as you were ever going to get them. This must have done something, as I won Gold, two seasons on the go in my division, using a club .44 Remington in the Cumbria postal leagues.
You will have times when nothing goes right, but it'll all come good in the end, then when you start casting your own bullets (if you haven't already), you'll see what a "Dark Art" it really is As an old newspaper advert used to say "I never knew there was so much in it"

Best of luck with your casting !!!

All the best
Baz