Not all barrels have orings on them. From my experience spyder barrels usually do have a oring to provide a larger amount of friction to stop the barrel from unscrewing itself during play. They can unscrew themselves because of the loose tolerances all around the entire marker and forces caused by the markers cycling.
A simple tank oring stretched over the threads may work for a while, but may need replacing more often.
I have a 2004 ,2/2003 Spyder Imagine's All 3 Rebuilt from ground up and not gonna lie the Biggest Thing when it comes to Accuracy is the Volumizer and the barrel . I have a a Smart-parts .689 Barrel 16 inch and its Damn accurate . I would Check what Size barrel Bore your using and what size paint balls and would recommend using a Spyder VS3 low Pressure Volumizer (its worked better then any after market one ive bought) and a Shocktech or Hush Bolt.
An aftermarket reservoir/volumizer isn't any different than the stock kingman ones. Only a few brands actually changed the inside volume from kingman's products, everyone else just changed the visual aesthetics.
Instead of a Low Pressure Volumizer, I'm pretty sure you mean a regulator that outputs a relatively low pressure compared to the older 800psi spyders. The regulator will allow for a better adjustment of the pressure on the paintball and the overall fps.