Cold mould or cold "melt" - ie: melt too cold to cast good bullets.

Usually, I pre-heat my moulds on top of the furnace, then flux the metal, then start casting. I've learned when everything is ready to go (should use a thermometer, but don't) and usually cast perfect bullets right off the bat.

Cold melt is much more often the problem than a cold mould, especially when you are pouring molten metal into it. The usual scenario is mould too hot, not too cold. I've never had a mould run cold.

Even my single cavity .50 cal mould will run hot if casting quickly.

This is a batch of .45 cal bullet fro my Sharps- from the first to last - good bullets - none to throw back.