Actually, headspace issues are secondary to ignition issues in CVA rifles for users of BH209. Their breech plugs are designed to be used with pellets, and as such are not 100% reliable using BH. After suffering several hang fires, and one misfire in my Accura, i began the search for a fix.
At about the same time sabotloader
posted about installing vent liner in a breech plug. This i did, and it cured the BH ignition issues. Before much more time went by, it dawned on me that a 5/8" x 18 grade 8 bolt would thread into the barrel of my Accura. It took me some doing, but eventually i made Accura breech plugs from them bolt. These plugs solved both the ignition issues, and the headspace issue.
The headspace of these plug is infinitely variable, and can be adjusted within 0.002" readily. At present the plug in my Accura was made to fit the STS primer, but can be changed to fit any longer primer, such as the CCI, W209, or the 209A. To fit a primer shorter than the STS, a new plug would have to be made from another bolt.
Then when the issues of the Accura breech plug were dealt with, CVA came out with the V2 and the QRBP. However, because of previous experience, the cure for the issues came quickly.
First thing was to eliminate ignition issues before the first trip to the range, by installing a vent liner. Then the headspace issue was examined. STS primer in my V2 grow 0.020" when popped.
Fix 1..............The metric o-ring
identified by Busta
can be placed underneath the STS primer in my V2 when using the OEM plug, and makes a good seal, and helps keep the anvil of the primer in the cup.
Fix 2...............I went to our local ACE, and purchased a small sheet of 0.005" shim stock and punched out shims using what we used to call a leather punch. I made 4 of them and removed and replaced the firing pin bushing with these shim in place. This made for a cleaner burn, and the primer only grew a couple of thousandths or so.
This V2 breech plug is now sized to use STS primer in my rifle. The fix was forced on me because the plug suffered flame cutting in the primer pocket. My fault because i kept shooting it when the flame channel had closed down to 1/16". A tiny piece of debris kept the primer from seating and allowed the blow back which cut into the steel of the primer seat. Still, there are some that think this type of fix for excess headspace in these rifle is the best.
A fella from Montana contacted me about his Apex QRBP. He suffered excess blow by, and had no way to reduce headspace by unscrewing the firing pin bushing. He say the Apex is not made the same way as the V2, and that one cannot reduce headspace by unscrewing the firing pin bushing. He also wanted the plug to be reliable with BH. He mailed me his plug, and i fixed him up, and mailed it back to him. He called and said our plan worked. How we were able to arrive at a workable headspace long distance, i have no idea, unless it was because he lives a good clean life. His Apex plug...................