Extremely unpopular opinion . I've pilled through ......I wanted to say tomes , but that's not accurate , I'll go with 1,000>< sets of shotgun data . I looked at cases , primers , and wads . I paired single cases , wads , and primers per load and looked for a specific difference that set a load apart by component .
When I started that was about 3 yr before the Federal 209A was dropped and they offered a more in line standard and a magnum only . The 209A in some loads and in several test examples as well as data demonstration would jump pressures 7-10,000 psi . Also at the very beginning of that testing which changed a significant amount of data .
The other contribution was base construction . Paper bases running 3.0 hotter wit Unique when you really look at it makes sense . The nature of it allows displacement of gas . The difference between a formed base in one piece with the case vs a separate base wasn't nearly as dramatic . As I recall like a half gr for equal pressures . The really outstanding difference was the actual shape of the base . Where an otherwise identical load in a FC steel case offered a 13kpsi and WIn and RP offer 12.8 and 12.5 kpsi a Fiocchi was running 9.9 and by the book the same speed . The FC was slightly coned towards the primer the Win and RP were basically flat the Fio has a raised cone above the primer .
Long ago , I want to say Whelen but it may have been Hatcher or another icon at Springfield Armory , there were a series tests done with extended flash tubes to get the fire up to the top/front of the powder column and keep all of the burn in the case vs half the charge entering the barrel . Things like 40 mm , 3"/50 and 5"/38 cased artillery shells are done that way . I suspect that ultimately it acts almost like a duplex load . The big thing was that they found cleaner burns , good news for the auto rifles . They burned slightly less powder for the same results or had gains with reduced pressures . The labor and cost however for use in the 06' didn't trump the gains . It did result ,I suspect , in a change to the shape of the GI case head inside . I haven't ever seen a US GI case older than 1941 , for comparison to a 1905 for example . The more current production has a raised cone above the primer . It seems to come and go in varying degrees .
I mention all of the above because the FC V shaped case bases would promote a full ignition in the base of the powder charge .
The flatter RP and WW tend towards a mushroom swirling effect I think .
The Fio has a full ring of powder below the flash hole and the flash is directed into center of the charge .
I can't say but applying these notions to a brass case with a thicker head expecting the flash tunnel to form a straighter narrower flash higher off the case floor should make for a decimalic improvement in efficiency over say some of the Russo cases that are dead flat .
All theory and conjecture of course as we don't have a window to see what is actually happening .