From The Manufacturer/Wikipedia/People Smarter Than Us:
The Smith & Wesson Military and Police revolvers are manufactured with scandium alloy frames with stainless steel cylinders with black durable PVD coating for outstanding durability and corrosion resistance. Capable of firing .38 S&W Special +P and the more powerful .357 Magnum® loads, M&P Revolvers offer double action only featuring the fully enclosed Centennial hammer and single/double action with the Chief’s Special exposed hammer design. From the Performance Center, the M&PR8 showcases a tactical rail in this large-frame, 5” barrel, eight-shot – the ultimate in accuracy, reliability and aesthetics.