So back in the fall I had posted that I was shooting IMR White Hots in my Accura with good results but wanted to try BH-209.I cleaned the barrel and breech plug completely before going to the range. I cleaned the fire channel with the drill bit as suggested and the flash hole with a nipple pick. Once at the range I loaded up with 90gr of BH-209 and CCI 209M primer. Third shot in I had a hang fire. Been seeing alot of post on this subject. Some say you must use the hottest primers, others say Win 209's are fine and won't foul the Breech as much. I recently saw the Russell Lynch video on the Accura and he was shooting the BH-209 with Winchester primers.I do know from using both primers in my Savage that the CCI 209M is a dirtier fouling primer then the Winchester 209 IMO.
Others say you must drill the flash hole to a larger diameter, others are offering aftermarket breech plugs. So what do you folks advise.

I also shoot a CVA and no I did not drill it out! But here is what I'm using. 295gr powerbelts with 100gr of blackhorn209 with cci standard 209 primers. The magnums were not giving me good groups.

use a #67 drill bit for a .032" flash hole or a #65 for a .035" flash hole.

Some rifles do perfect without mods but some rifles also have primer depths that are a bit deeper and allow the primer to blow back into the frame area and lose a good deal of heat from the primer.

Also I have found that I have to pack "seat" the bullet hard or no go bang!

You didnt say what bullet sabot combo you are using but from what I here BH 209 likes a tight sabot. And as has just been posted packed tight.

I am using a Barnes TEZ 250gr flat base. I replace the supplied Blue Sabot with an MMP-HPH-24 which provides a pretty tight fit.