Abstract: Tests are made with three different piercing plug cone shapes to identify shell gauge changes during secondary piercing process.The result shows that the plug cone shape design ed according to the formula related to specific deformation conditions acts well in reducing the shell wall thickness nonuniformity,which is in close connection with the fact that in the second section of the piercing zone,the unit screw-down a mount imposed on the shell being pierced and the shell axial elongation are smaller and more uniform.