Depending on the caliber your MAY need to resize. The new brass MAY not be perfectly dimensioned for your gun. After it is formed it is polished. It MAY get deformed during the polishing process. Some calibers are thicker or thinner than others. For example, .45 ACP I would not worry about it as it is fairly thick. .44-40 would need to be sized as it is very thin and easily deformed. The resizing should not be difficult as the brass is very close to proper dimensions already.