both my 40/72 and 45/2.4 have proven to shoot well with a case droptubed full of black powder and about a 1/8" airgap.
this is using bullets patched to bore diameter or 0.001 bigger.
the question is whether filling that airgap with a greased felt or other wad would affect accuracy or not.
the idea of this is to transfer full shock to the bullet, maximizing bumpup in order to get the best confetti to clear the bullet a.s.a.p.
I have been told that it would be better to seat the bullet further in, use the wad as suggested, and retain the airgap for best accuracy.
keep safe,
bruce.

Bruce I have a few rifles that I shoot breech seated PP bullets in.  My findings are much like yours, assuming I understand correctly, in that the bullet is seated pretty deep in the chamber.  More so than with a groove diameter bullet.   I use a 0.030" wad to hold the powder down, and seat a 0.060" LDPE wad with the bullet.  There is always a significant air gap of 1/8 or so.  I tried briefly in one of my .32-40's to fill the airap with a felt wad.  There seemed to be no change in accuracy so I stopped doing that.

Chris.

I have often wondered if it is the length of airgap that tunes the load, or the relationship of the base of the bullet to the leade/transition.
sometimes you think you have arrived, but when you look up this usually means that you have only arrived at the bottom of the mountain and you have to start climbing.
keep safe,
bruce.

chris,
that is my suspicion also.
getting lead bullets across transitions and leades is just a variable that you eliminate.
really it does not matter why.
as long as the results pan out as hoped.
I am surprised that no big names in breech seating are chiming in here as hoped.
keep safe,
bruce.

chris,
that is a pity.
it was done and understood in the day to the extent that it was not written much about due to the assumption that it was normal.
if more people tried it they would see the great improvement in accuracy over fixed ammo that was taken advantage of by both scheutzen and creedmoor shooters.
the most probable reasons modern people avoid it are both urban myth and it makes brains hurt.
keep safe,
bruce.