First Strike (Tiberius Arms) Factory replacement part: ORNG 017-S70
replaced it one and done
I bought my t15 new and after about 4 uses from the factory, the o-ring started to chip away. Granted it still worked, but I was not looking forward to a failure on the field. I purchased several of these and they are still sitting in my box. Its good to have replacements, because you never know when the parts will disappear, however I replaced my o-ring once and a year later I am still using the same ring. I play at least twice a month if not more. Great buy!