European shotgun primers are slightly tapered.
Winchester primers have a reduced diameter straight section at the lead which acts as a lead-in and positions the primer in a readied (perpendicular) position prior to the action to press the primer into the pocket. This requires a little more attention to detail relative to the assembly sequence in that the act of seating (pressing) should be done in as straight up & down action as is possible.
The European design is one that allows the pressing action to be slightly off center and is reliant upon the taper to align the primer properly once seated.
Just two different ways to skin the cat that results in deformation of a primer pocket making interchangability a not so feasible proposition.
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This ignores the possibility of blowback and slightly large chambers.
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Threaded Mode