- The HK SP5K Folding buttstock is constructed from durable black polymer. This factory original, German-made HK stock is designed to fit both the MP5K and SP5K pistol/rifle models are chambered in either 9mm, 10mm, or .40 S&W. This folding buttstock with butt pad lets smaller stature shooters or officers wearing body armor have a weapon that is better fitted to their needs. The foldable stock replaces the factory butt cap and folds to the right side of the firearm. Heckler & Koch’s HK SP5K folding buttstock is world-renowned for its high quality. You get the highest quality materials and assembly when you buy an H&K part or accessory.