MAS RHOP 2.0 Bucking and Concave Nub
1. Disassemble and clean the existing barrel group
A) Set the hopup dial to 0 and pop the arm pin out with a narrow punch. Push the arm away from the dial to unseat it.
B) Use a small flat tool to pry the corner notch of the barrel c clip downwards to unlock the barrel. Remove the barrel with old bucking and clean as necessary with isopropyl alcohol.
2. Install Barrel + Bucking
A) Apply a very small amount of thick (not runny) grease around the circumference near the barrel window without getting any inside the barrel. Slide the bucking onto the barrel while pinching the top of the bucking to avoid the contact patch scraping against the barrel window.
B) Insert the barrel with bucking into the hopup chamber and reinstall the plastic c clip and brass stabilizing ring. Ensure that the alignment groove on the barrel is facing downwards. Gently twist the barrel to ensure it’s seated correctly with the c clip. The barrel should be firm without side to side or back and forth movement, and without rotation.
3) Install Nub + Arm
A) The slot for the nub will now be visible through the top of the hopup chamber to provide pressure to the top of the bucking contact patch. Drop the nub into the slot and place the arm over it while locking the back of the arm into the dial groove while set at 0 hop.
B) While the arm makes contact with the dial and the nub, apply slight pressure to the arm to align the hopup unit and arm pin holes and reinsert the arm pin.
C) Before inserting into the upper receiver, apply about 25% hop and check engagement by looking down the barrel group. You should see a slightly protruding crescent moon shape that’s uniform on both sides.