This double o-ring piston head is machined from the structural plastic TECAFORM, also known as POM.
 
Nine holes are drilled on the front side to allow the negative pressure to quickly equalize, reducing friction on the rear stroke, and helps expand the o-ring on the forward stroke. 
 
Using this material and clever design was reduce total weight only to 5,4 grams. This low weight increases the lifetime of the mechabox and also increases fps.
 
We recommend limiting the use of this piston head to an M130 spring.
 
Advantages:
Minimum weight (only 5.4g)
Perfect sealing
Plastic nut and bearing that reduces total weight