I think this thread belongs in handloading, as there isn't necessarily a gunsmithing issue involved, so I'll move it for you.
Winchester sometimes has pretty awful wall thickness variability and the thin side will expand more than the thick side; what the late Roger Johnston called a banana shaped case from the slight longitudinal curvature that often also results. Unless you get a NECO gauge
or the RCBS Case Master
, both of which use the chord anvil Roger Johnston designed for the NECO gauge originally (I think the patent expired, which is why RCBS now has it), I don't know a good way to see wall thickness runout back near the case head. But that's what you want to measure.
Unless you know your chamber is oval, the brass wall thickness seems like the most likely critter.