The disclosure is directed to improvement in piercing and reeling mills, to facilitate the changing of mandrel plugs and mill shoes. In piercing and reeling operations, mandrel plugs are supported at the ends of elongated mandrel bars, and workpieces are driven over the mandrel plugs while being rotated, to effect piercing and enlarging, in the case of piercing mills, and to achieve roundness, in the case of reeling mills. In each case, after processing a finite length of tubing, and while the workpiece remains on the mandrel, the mandrel is retracted, first stripping off the just-used mandrel plug, and then withdrawing the mandrel from the processed workpiece. A multi-position magazine is provided for retaining a circulating supply of mandrel plugs such that, after each operation, the used mandrel plug may be returned to the magazine for cooling, and a new plug may be brought quickly into working position ready for the next operation. The arrangement of the invention provides for expedited loading and discharge of the mandrel plugs, for greater efficiency in mill operation.The invention is further directed to improvements in mechanisms for changing of mill shoes and mill shoe adapters, as is necessary from time to time in conjunction with changing of the size of the workpiece processed. The improved arrangement permits an upper mill shoe to be released and supported on the lower mill shoe, and includes provisions for horizontal transfer of the lower shoe, along with the released upper shoe, to a position clear of the mill rolls, where they are easily accessible to an overhead mill hoist. Provision additionally is made to facilitate and expedite the removal from the top of the mill of the entire mounting chair for the upper mill shoe and its adapter, to facilitate change of the adapter and chair for different sizes of workpieces.