I had a partial case head separation using a 1.8oz buffer

[DeNinny]

If unsupported area is radiused into supported via feed cone, wouldn't we see some evidence of the brass following the radius rather than be straight to a fissure with only the lower half exhibiting extrusion? I can see a tear causing such a clean slice, but an in battery blowout should be a bit jankier on both sides of the failure point.

It's an interesting thought.... both sides of this, where the only difference really is 1 microsecond of time.

Remember that the other half is supported by the chamber wall, so only half becomes "janky". The half that is unsupported which again is under the feed cone area.