Prometheus Piston head for v2 next gen recoil m4 etc
High quality upgrade part for the Next Generation Recoil system
Fits M4 sopmod / HK 416 / HK 417 etc

Info from prometheus below ( direct translation)

Use excellent POM material which has excellent fatigue resistance and abrasion resistance friction characteristics, excellent self-lubricating performance, in head shape which enhances operation at the time of air supply, fuel consumption efficiency & airtightness at the time of compression, exhaust efficiency and durability. Moreover, by equipping the thrust bearing which seemed to be difficult to mount in that space, we eliminate the yore at spring compression and maximize the performance. It is an essential item of internal custom.