O-Ring on barrel

If it broke then it was deteriorated. The "O" ring is there mainly to act as a friction lock to prevent the barrel from loosening. There is no harm done without it, but you may have to re-tighten the barrel  every few minutes of play. But, barrels can loosen even with the "O" ring in place.

BTW, "O" rings are cheap and can be gotten at virtually any hardware or automotive parts store. If you want to spend money, you can get one of the "O" ring "parts kits" available for your marker. I keep two or three barrel size "O" rings and bottle nozzle "O" rings in my parts box whenever I play. Having those on hand will assure that you will never have to replace them at the field.