Cva has a shim kit on their web page under the parts page. It shims the firing pin bushing out closer towards the primer for proper "headspace" ...it stops the excessive primer blowby and is a permanent fix rather than use the "oring trick"
This is a direct replacement plug for many CVA muzzleloading rifles. It was designed for use with the DIS priming modules and uses large rifle magnum primers to give you a hotter ignition and a 100% leak free seal. No more worrying about the rain getting into your 209 shot gun primers. With this...
Just something else to consider and may give best performance, but it's obviously comes with added cost.
If you have not already done so go to the BH209 web page.. They have a lot of good information. I do all of my big game hunting with a couple of Knight Disc Extremes with their bare metal primer breech plug. You can get a set of welding tip cleaners to keep the flash channel clean. With the Knights I also use a drill bit to clean out carbon build up in breech plug. I weight all my loads due to lack of consistency in BH209 between lots. I had a few issues using the muzzleloader version of 209 primers so I switched to Federal 209A ones and no problem. If you are getting blow by on primers you may need to shim the breech plug. The various brands of 209 primers vary in length with the Winchester being the longest. Lots of info at modernmuzzleloader.com. My favorite load is 110 gr(V) of BH209. 300gr. Harvester PTGold bullet with their Black Crush Rib sabot and Federal 209A primer.. 1/2 inch MOA at 100 yds. with my Long Range Hunter and deadly.