My favorite style of LR is the Bucks County which are known for being very petite so in order to make them as petite as the originals or even more so, the following are done and have also used it for building slim Lancasters. A "B" weight Rice swamped bbl X 46" lg is used w/ both styles.
Starting w/ a blank, which is sent to Dave Rase for the bbl/RR work and the webs req'd are 1/16" max at the breech and 5/32" at the muzzle...Dave does a bang up job w/ these thin webs.
The bbl lug closest to the breech has to accommodate a 1/16 dia music wire pin w/o intruding on the 3/8 dia RR hole and is different than the other 3 bbl lugs which are inverted, dovetailed "T"s. See pic. Needless to say...very accurate layouts have to be made for this pinhole and also for the #6-32 front lock bolt which has to miss the RR hole and requires a shallow groove in the bbl.
To obtain space for the #6-32 front lock bolt tapped hole in the lock plate "nose", a high location for the 1/4 dia White Lightning TH liner is req'd…..the one shown just misses the upper corner of the side bbl flat. A Chambers' Late Ketlnd is used. W/ the Chambers' Golden Age, the White Lightning isn't as high.
From a side view, the following don't make the LR look more slender, but I do it anyways...the top of the forestock is quite a bit below bore center....this makes the thin wood alongside the bbl more sturdy,1/3 of the RR is in the groove , 3/32" of wood is below the RR hole and the entry pipe drop down from the finial to the pipe is .040 { sheet thickness} . Because of the web thicknesses , both bottoms of the lower and upper forestocks taper towards the muzzle...this was an unintended nicety caused by the web thicknesses.
To achieve a very slender BC or Lancaster, very accurate layouts are necessary and I hope this all helps.....Fred