I think you’re mixing terms somewhat. The recoil reducer in the stock is called a buffer. Piston systems are used as a replacement for the direct impingment method of cycling the rifle.
There are a fair number of buffers of various weights to tune the recoil. Hydraulic buffers offer the most recoil reduction, but are generally used on full auto rifles.
I recommend you go to AR15.com to research the topic. If they can’t answer your question, there is no answer.
I’ll admit to mixing the terms, as I realize what you are saying.
this device goes in the stock, has a gas-operated piston instead of the usual spring, and reduces recoil.
The video narrator refers to the device as a buffer. You are correct.
The video shows the device I intend to order/ try.
Going to AR-15 is like talking to pilots in a FBO hangar. ;-)
I’ll stick with my friend’s advice. He’s the real deal, and I know him personally as such.