I have a HK MR556 Upper that I am using on a KDW Lower with a Sliding MAGPUL Stock. The new upper came with a HK TUNGSTEN Buffer and Buffer spring. I switched out my M4 Carbine buffer and spring and installed the HK Buffer and spring. My question is can I use a M16A2 Standard Stock buffer spring spring as a Back-up spare? They look about the same length/ The HK Buffer spring and the A2 Buffer spring. My M4 Spring is shorter than the HK Spring and the HK Manual says to use their Spring only, but they are $60.00 plus dollars and the A2 Springs are Like $5.00 TO $10.00 DOLLARS. I am getting a Spare parts Box together for the HK UPPER, I have a lot of M4 Spare parts for my other Rifle. So will the Other A2 Spring work? Thanks:wink: