This has been my rule of thumb. I usually get 2 molds going at the same time. Both will be either cast, or aluminum. I figure, that way, their both going to stay at the same temperature. Not sure if you could get one iron and one aluminum going at the same time, as the aluminum mold would cool off, just a little faster than the iron mold, and might throw your timing off. I'm not sure if this true or not, as I've never tried it. Maybe somebody will enlighten us. As far as the number of cavity's per mold, all but my muzzleloader molds are 2 cavity, and they are one cavity.But I am casting for revolver and a couple of rifle calibers, gas checked design. I was more interested in quality, than quanity. But then, I'm not casting for my .45's/.40 or 9mm's. There are several casting company's that make pretty good bullets for the auto-loaders, and for what they charge per 1000, I have a hard time justifing my time, on a lube-sizer doing one bullet at a time. That might change in the not to distant future, what with metal price's going thru the roof. And if that happens, I'll probably try a Lee 6 cavity mold set up for tumble lubing. Time will tell. gypsyman