At the risk of writing something your friend already knows the Winchester Apex and the CVA Apex are not the same rifles even though they are/were made in the same factory by the same parent company. Be cautious when buying parts that you have what you really need.
I owned a Winchester Apex for 4-5 years (recently sold it) and used it with BH 209 for 2-3 of them. The original Apex breech plug had the screw slots in it. Get rid of that and get the later CVA plug with the hex head. It is easier to remove and install and there will be a lot less crud get past the primer as it will be fully surrounded. That crud equates to flash energy that didn't make it to the powder column. BH is harder to ignite than some powders and you want all the flash heat in the powder, not as crud in the breech. If there is crud in the breech area you are wasting energy and raising the risk of misfires.
The original plug has a flash hole of around .020" and for BH209 you want something in the .030 to .040" range. Small drill bits can be hard to find and you may need to order both the bits and a pin vise online (I used Fastenal). The pin vise is needed unless you have something else with a tiny chuck since the bits are too small for a standard chuck. I got a pin vise that has a 1/4" hex shank on it so I could use it in a drill. I chucked that up in a heavy duty Dremel tool for the job.
I use an .035 hole and have very positive ignition.
It is also important use a drill bit, turned by hand-not a power drill, to clean the flash channel from the chamber end of the plug back to the primer end. See the link from the BH website:
http://www.blackhorn209.com/specs/breech-plug-cleaning/BH209 also wants a fairly tight fitting bullet or sabot bullet combo. Without a tight fitting projectile to contain the powder ignition will range from spotty to nonexistent. If you put the butt of the rifle on a scale while ramming a projectile and you see less than 20 pounds or so of pressure needed you will have a much greater risk of misfires. Most folks who check these things like to see 30-35 pounds of pressure needed. There are a lot of different thickness sabots available from MMP Sabots to help you get there and you'll save money buying loose sabots and bullets as opposed to buying prepackaged ones.
I hope some of this helps and good luck.
Lance
