We at Umbrella Armory use O-Rings to space the Hop-up against the gearbox for tight airseal. Compared to springs, our O-rings allow for significantly higher tension and finer-control of pressure to ensure there is no vibration or air-leaks possible. This is especially important for high FPS or high cyclic rate setups in which low tolerance control of air pressure is required. The number of O-rings required can vary depending on the manufacturer of the outer barrel and trial and error testing is required.
Tip: Due to the large amount of variation some manufacturers have even within their own model range, we strongly suggest to do a test installation with no o-rings installed. Once the barrel is dropped in, reconnect the upper and lower receivers together and test the movement of the barrel from the ejection port, or from below the magwell move. If movement can be detected, add additional O-rings until the Hop-up no longer moves back and forth. When you are done there should be slight resistance when closing the upper and lower halves together and pressing the front pin in. Carefully rejoin the upper and lower receive and do not hammer the front pin too hard, risking to break/crack the lower receiver’s front tab.
Standard O-ring values:
Krytac uses 3 O-rings
G&P uses 3-8 O-rings (depending on outer barrel model)
VFC uses 1 or 6 O-rings (depending on outer barrel model)
Sold in quantities of 10.
Or you can buy 100 for like $4 here: https://www.mcmaster.com/