Good testing Ron. It is good to see these tests done by an unbiased source. Thanks for doing it.

Grouse, let's assume there is a pressure difference. But if the group size is the same, velocity is the same, and breech cleanliness is the same - who cares? One is as good as the other.

The assumption is with that particular load and primer you are correct. But as you progress up the powder scale and projectile weight especially with BH the pressure becomes more significant, especially if you are not using a primer that has 0 head space. My point and only point about pressure on the nose of the primer is the dome design does reduce this pressure to some extent, as also does the volume capacity of the flash channel.

Also as an added point not all guns on the market have or are set for zero head space - then the blow back pressure becomes even more significant - not necessarily because of safety but cleanliness of the breech area.

Even in a closed breech the difference can been seen if you are not using the same length primer from shot to shot - again effecting only cleanliness not safety.

A lot of people seem to prefer the Remington STS which is a shorter primer and unless you set your rifle to 0 headspace for each individual brand of primer cleanliness will be effected. Do all people have the ability to set 0 head space for the primer of their choice?

To my knowledge, in all of the forums, and blogs, on all of the internet, and all of the world wide web, there is no empirical data to support that statement.

As long as the primers are not sticking you are exactly right. Different primers have different results. But either way, both ventliners work. I dont know who said they didnt.