Airsoft GI DIY – Installing a New Hop Up Unit (M4 / M16 one piece hop up)

We received a lot of requests from customers on how to do some minor modifications / Upgrades to their airsoft guns. In the Airsoft GI DIY series of articles, I have Marq install minor modifications to standard airsoft guns that most airsofters can handle. These are very basic operations, and do not need to open up the gear box. Although these modifications are considered to be simple, if you do not feel comfortable, always seek technical help from our Tech Department.

Disclaimer: This is only a general guidance on how to install a new inner barrel on most Airsoft Electric Guns on the market. If you do not feel comfortable, please seek assistance from our Airsoft Smiths who are qualified to handle all tech works. Airsoft GI IS NOT responsible for damage inflicted on your airsoft guns by following these steps. Thank you.

Parts required:

  1. New Hop up unit
  2. Small Phillip Screw Driver
  3. Small Flat Head Screw Driver


  1. Purchase a new Hop up unit.
  2. Install the hop up knob on hop up arm
  3. Install hop up knob and arm into the main hop up housing, and install the rear containment pin.
  4. Install retaining clip on the opposite side of the housing by pressing down with a flat head screw driver.
  5. Install hop up arm spring.
  6. Apply slight downward pressure on the hop up arm and Install adjustment gear into the Hop up unit.
  7. Install Middle gear
  8. Install retaining clip for the middle gear by pushing down with a flat head screw driver.
  9. Install main adjustment gear.
  10. Install set screw for the main adjustment gear
  11. Install the gear box pressure spring
  12. Install hop up bucking, barrel spacer, and inner barrel, please click HERE.

Special Notes:

  1. The black circle plate below the C-Clip is for your M4 body. It is installed on the opposite side of the M4/M16 Selector Switch.
  2. This Systema 1 Piece Hop Up Unit comes with 3 different sizes of barrel spacer. Every gear box and inner barrel dimension is different. You want to install a spacer to have little to no play between the hop up unit and the gear box. You want to have the hop up unit sit against the gear box as close as possible. This will improve the velocity and the consistence of your AEG.
  3. The white part is the C Clip, please refer to Step 12.

One Reply to “Airsoft GI DIY – Installing a New Hop Up Unit (M4 / M16 one piece hop up)”

  • Looking for info on Masada GBB. I have one. Love this beast but looking forward to some performance mods. Any idea’s?

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