Available now. HK91 A3 collapsible/telescoping stocks have a fairly stout felt recoil. I have developed a new piston and spring assembly that reduces the recoil up to 30%+. Works on any HK A3 stock attached to a .308 rifle, even the HK51 and Vector V51.
Above is the prototype installed. Below in the picture at the top is the improved piston and spring, next is the HK A3 stock buffer and spring. The next item is the backing plate that retains the buffer assembly. This contains a small spring and piston that applies pressure to the stock in its closed position. It will need to be shortened slightly to allow full travel of the longer piston.
The piston is made by Rim Country Manufacturing, a leader in US made HK parts. The new piston is 40% longer and is hardened to 55-60 Rockwell. The recoil spring is about 30% lighter than the stock spring. Price runs $45 delivered for the new redesigned piston and spring. I can also supply and install the new buffer system into your A3 stock for $89, return postage included.