So now you are making me wonder if the ones that don't hit the stop were all driven from the top, and from the bottom it's a bigger diameter.
If the bottom bore is larger and the top bore smaller to make a ledge as the stop.
That could mean the plug ends up getting resized by the smaller bore through the top and ends up loose in a slightly larger bore.
Hmmm
Now I have something to add to the "Gotta find out the details on that some day" list.
