Larger locks, like the big german lock in the photo, or large English locks the fence can be set equal with the breech and place the vent just in front of the plug, if the plug is 1/2". Smaller locks, like Silars and such, will put the vent into the breech plug threads if you line up the fence with the end of the barrel. Haugh would file a deep groove in the plug that flarred out to toward the edges of the plug, then polish. I never remember him shortening barrels and plugs to avoid this. Many originals used this same approach.....So I guess it is preference, but I prefer to place the vent just in front of the plug face, and the lock falls where it does....My 2cents.