Do 209 primers made for muzzleloaders have a different diameter than the primers for shotshells? I seem to be getting some blow back in my Winchester Apex around the primers I amusing -which are Winchester 209 shotshell primers. Or is this a possible oversized opening in my breechplug??

Regular Win209s and WinT7s are pretty close in length. Both are a little longer than other primers but a tad more narrow. Some Euro primers like Fiochis and Nobels are a fair amount larger OD than US brand primers.

Its possible a new breach plug could help. You might also try cleaning the flash hole in the breach plug. When the channel gets too much carbon, pressure on the primer increases.

I think a 3mm bit will fit the Kodiak plug.

Here's something that could be useful to you and others.
(BP)