The orings actually have noting to do with efficiency or "loss of air."
Some of the older Tippmanns that did not have the oring, that barrel adapters are milled differently to accept the oring(or not to), had issues with them rotating out even when the front screws were tight.
To fix the issue of barrels falling off from the adapter Tippmann added the oring to essentially take up for space to account for the varying tolerances of the parts.
So if your gun does not have one, and the adapter does not rotate freely you don't need to change it. If your gun does have one, good news!
If you need one check your local paintball store, I am sure they will be happy to help you out.
Any questions feel free to PM me.
Certified for: Planet Eclipse, DYE, Empire/Invert, Tippmann, BT, Bob Long and Dangerous Power with 13+ years experience in the industry, ask me anything, I know more than anyone.