Used the metric O rings that came with the Western Powders breech plug for the two guns I ordered them for. One was the Accura V2 and the other Petes Wolf. I had no need for these O rings on those guns since you can adjust the head space with the shim kit.
The Apex however can not have its head space adjusted since there is no firing pin bushing. I installed one O ring yesterday, took 3 shots at 50 yards and the primers and O ring looked excellent. I will keep track how many shots i can get out of the O ring using the CCI 209 Magnum primers and report back.
I do plan on using an O ring on my Traditions Ultra Light LDR once she arrives.