I'll give it a try !
Graybeard , or anyone when we shot the 7 TCU we made cases for it by starting with loaded .223 rem , since the contender head spaced on the rim since the extractor snaps up in the groove much like a 1911 does and holds the case in the correct position to fire we did just that and it expanded the case mouth most of the way to 7mm , making the sizing to 7 TCU much easier and with less damaged cases .
Is it possible to do the same with 307 Winchester in the 356 Winchester ? ( i have around 100 307's on hand with no gun ! )
anyway the case for both are the same except for case mouth ,