Couldn't recall if i ever informed you what i think the issue is. It appears the problem was the primer socket was too wide as you suggested. Somehow i was able to do a nicer job milling the primer socket on the second plug, and it ended up quite round, and some smaller than the socket in the first plug. Been shooting it a lot, and it doesn't let soot past the primer, like the first plug did. Following is a photo of the second plug, at home after a session of shooting.
The black band at the right isn't soot; it is somehow the result of the flash.