Prometheus POM Piston head for Scar next gen recoil V4/9 SCAR L / H
Info below from Prometheus (if it makes sense)

Aiming further for air supply efficiency and exhaust and operation efficiency during compression, it appeared in a new form superior in durability. Material is also rich in fatigue resistance, abrasion resistance friction, use excellent POM material with excellent self-lubricating performance. Moreover, equipped with a new thrust bearing which seemed to be difficult to mount in that space, we eliminate the yore during spring compression and maximize the performance.
It is an item that matches high cycle from low rate spring.