I suppose you could use wheel weights, but you'd get tired of lugging the big hammer needed to start them down the bore. Whack! Whack! Whackwhackwhackwhack. Cuss. Whimper. :rotf:
I've had the same experiences highlighted by everyone else. One thing I'll add, you want a good hot mold and hot lead, and you DON'T want any wrinkles or flaws of any kind for best accuracy. It's one of the few bullets I weigh and sort.
The ones with wrinkles and such are fine for plinking, and you can expect a few at the start of each castin session. But if you cast as many as 20 and are still getting wrinkles, crank up the lead heat and wait a bit longer. You'll still get a few with wrinkles until the mold heats up completely.
One thing that helps in pouring is to concentrate on making your pour a little off-center to the sprue hole. Folks explain that it causes the lead to swirl more and fill better. I don't know about the swirl part, but I do know I get fewer rejects with an off-center pour.