The data i collected was done alternating the vents. That data showed no significant difference between them vents. Your data was collected using one vent, one vent. How is it relevant? How is data collected using one vent relevant to comparing different vent shapes?

Also, the validity of your conclusion is questionable. Somehow you are able to conclude that the domed vent is better, because the last shots from a warm rifle/breech plug, are a higher velocity. To me that seems to be a non sequitur. It is only natural the last shots over the clock from a warm barrel/breech plug will be faster. This shows up in my data too.

Perhaps the reason you don't have the ability to explain why the dome shape reduces back pressure, is because it doesn't.