It adds some to the dia,the longer you leave them in solution the bigger they get. You need to acid wash the slugs to etch them and remove ALL the oil/lubricant before starting. You need to heat the solution for best results.Copper sulphate is VERY poisonous and must be treated as such. You will need a power source like an old train controller. Does it work ? yes! Is it worth the time and trouble ? NO! It is actually cheaper to BUY them if you value YOUR time at mijnimum wage! Do a search for plated cast bullets to see the difference. Then there is the fact that I DO NOT WANT plated bullets! that's why I cast! OH ,almost forgot, distilled water works best.