It is an Action Army AAP-01 short stroke buffer with all the accurate dimensions. The uploaded files contains a buffer with 5mm thickness.

I attached a step file too, it can be customized easier to the specific thickness.

I chose 5mm, because it has good effect on firing cycles, but also spare the internal parts from fatal damages.

(On the picture there is a silicone rubber buffer layer, I am working on the molding.)