I've read several articles on this and "walked away" with two different answers. The wheat cents with copper used from old shell casings was it three years consisting of '42, '43, and '44? And all the mints used this copper during these years?
Despite sources to the contrary, the experts (and Doug) say that the shell casings were never used. http://www.cointalk.com/forum/t44597-2/#post476499
fyi, the 1943 wheat pennies were the zinc plated steel pennies (sometimes called silver pennies by newbies)