Almost explained
Full wadcutters are cylindrical and to protect the fragile corners, all of the bullet is usually contained in the case. The bearing length of such a bullet shoved that far into the case is quite long and if it weren't either tapered to smaller than .3575" or made hollow based to allow for deformation, it would bulge the case.
For target work, a loose fitting bullet is patently objectionable and a tight fitting case is deemed helpful. Bulged cases are problematic especially in tight chambers HBWC's to the rescue.
These can be deep seated yet expand to fill the rifling, keeping fine accuracy. They are usually soft lead and do not withstand anything more than light loads.
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