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I have determined that the method documented in this article is a bad idea. See the "Aftermath" section below for details. The better solutions are the Bravo and Lonex hop up chambers. See the "Bravo Airsoft M4/M16 Metal Hop Up Chamber Review" article.
After getting a Bravo Airsoft M4/M16 Metal Hop Up Chamber (see photo below) in my APS ASR106 Mini Patriot M4 airsoft rifle, I have became a believer of hop-up units that has the o-ring in the back to seal with the gearbox.
But my other APS ASR107 M4 URX Raptor rifle already have metal hop-up unit. It would be a waste to just chuck it and get another Bravo Airsoft M4/M16 Metal Hop Up Chamber. I do have tons of o-rings though. In fact, box full of them. I have tried them on the stock APS hop-up unit (see photo below).
The 10mm ID x 14mm OD x 2mm W o-ring seems to fit pretty good.
That was an easy installation and hardly cost anything. A box of o-rings is less than $10. You can get a box of one size for a few bucks at the local hardware store.
The photo above shows the hop-up unit sealed against the gearbox. The o-ring is between the hop-up knob and the version 2 gearbox. There is no contact between the o-ring and the hop-up knob. Even if there is, it would just hold the hop-up knob in place better.
While building my APS M4 URX Raptor into a DMR, I could not get the velocity over 318 FPS. This gun has chrono'ed at 359 FPS before. No matter what inner barrels or springs I put in, the FPS has stuck at 318 FPS. And I have sealed everything completely.
Long story short, it turned out that by introducing this o-ring between the metal hop-up unit and the metal gearbox, it created a gap between the air nozzle and the bucking. After removing the o-ring, suggested in this article, the inner barrel and spring modifications caused the gun to shoot at 455 FPS.
In hindsight, don't short change yourself by rolling your own hop-up mod in this manner. Purchase the Bravo or Lonex hop-up chamber with the integrated o-ring for perfect seal (see "Related Links" below).