Shell casings were used in 1944, 1945, 1946. All cents coined in those years were struck from recycled shell casings. They are struck in brass (95% copper, 5% zinc) rather than bronze (95% copper, 5% mixture of tin and zinc). The brass coinage was a vast improvement over the zinc-plated-steel 1943 cents.
The shell casing cents had high mintage figures, are extremely common, and command no premium because of the slight composition difference. Some of the coins show slight coloration differences because of the increased zinc percentage and lack of tin in the mixture.