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Hey, I have an IcS m4 (221). The hop up unit is proprietary along with pistol grip which is why you find yourself in your current predicament. If you have another hop up unit for m4 try that with the mk18 barrel. It might work and you can test to make sure you don't have compression issues with air nozzle and different hop up unit. I have not tested this as I use the standard IcS outer barrel.
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