Your cheaping out, that is why you're sore and inaccurate.
This is what you need to pure cleanly:
http://www.do-itmolds.com/product.aspx?i=257
You can do it for nothing...and melt lead with a ladle on a stove but I don't recommend it.
Two schools of thought...either buy what you need completed (ie, 100 packs of tail weights) or get the right stuff to do it yourself.