Nylon construction but very sturdy, comes with all the fittings. An excellent price/performance hop unit. Arm pin is metal and rotary wheel has the right amount of tension. Includes a smaller barrel spring and a free ZCI shore 60 silicon bucking and nub.
A worthy addition is an over-the-barrel tension spring as well:
https://www.ak2m4.co.uk/hop-up-chamber-tension-spring-large
I recommend using a tiny amount of silicon grease on the wheel where it meets the oring.
Unlike many cheaper hop chambers on the market which use a molded nub on the arm the ZCI version uses the concave style design, good for Omega nubs etc.