I have an Oly unit. Very well made, guaranteed for life. And it does do more than just get rid of the spring sound! It softens, or rather smooths out, the felt recoil; it also reduces the violent impacting action in the bolt group, more than likely extending the life of these parts. It is also adjustable via a set screw made into the front of it, to adjust the "feel" of its operation for each individual shooter. It is pneumatic - not hydraulic. It is very similar to a gas-charged shock used on a car. From what I have heard, Oly tested these VERY rigorously, and made sure they don't fail, before releasing them; however, do like I did - save the old buffer & spring, just in case....that way, you can use them if you ever have to send your pneumatic unit back in for FREE replacement! It just slips right in and out, just like the stock spring and buffer.