At Camp Edwards, MA I saw a photo of a M1 helmet where a .223 bullet fired accidentally had penetrated the steel pot, gone round between the liner and the pot, and exited the rear.
The shell of the British RAC [tankers] helmet is the same as the paratrooper’s one, and the Don-R [motorcyclist] 's one. Only the liners and the attachment points differ.
In recent years fake para helmets have been made by welding up the screw holes in the other types of shell and drilling new ones for a reproduction liner.