having had real success with this in both 40 and 45 cal rifles, its accuracy potential has become obvious.
a case droptubed full of powder such that the wad is level with the top of the case neck, and then start playing with the airgap.
this gap has proven to be about 1/8" in the above rifles, but shooters of smaller calibres report success with shorter gaps.
when you hit the gap zone of correct tune it is very obvious.
a good wiping regime is required to capitalize on the potential locked up here.
in this case it has proven to be a religiously clean and dry bore.
a point of interest is that breech seating tuning seems to be about the relationship between the base of the bullet and the throat angle more so than  the actual gap itself.
I thik I have proven this by seating bulletsnormally, and then filling the gap, and/or part filling it, with things including felt before chambering the case.
point of impact of group, and group size seem totally without difference.
others with more experience please comment.
keep safe,
bruce.