Bruce,
Before you invest in the o ring, make sure you can use one. I'm not sure on your setup. My rifle (wolf) doesn't have enough slop to allow me the use of one. I only have a .020" gap on 209 head spacing. Not near enough for an o ring. If you don't have sufficient room then the only other option is the longer primers or the shim kit from Cva for the firing pin bushing if this is what you shoot. Someone posted on primer length and which ones were the longest. I can't remember which ones they were. I shoot the T7 209 which are reduced power. With a tight seat on the primer there is no blowback
Dan