ronlaughlin
I believe you are may be partially incorrect in both of your assumptions.
It seems to me that efficiency of heat and gas go hand in hand with velocity. Wouldn't more efficiency lead to higher velocity?
Please look at this target shot to check velocity changes with different primers. There primers all produce different burning temps and volumes of gas. Compare the velocity difference between them. One thing that helps this test, as Lee has already told you on another forum, is the shape of channel in the Lehigh Vent Liner which allows better efficiency for all primers.
If you look carefully you will see the weakest primer of them all a Remington 209-4 (a 410 shotgun primer) actually produced the best velocity. It does not produce as much gas volume as the other primers and it is able to transfer through the vent liner because of that. Bottom line is it still ignites BH just fine and with a very slight increase in velocity.
I do not have the abilty to explain to you why the dome shape helps reduce the blow back pressure through the Ventliner and BP to the nose of the primer. I have tried before and have failed at a very simple principle of 'angle of incidence = the angle of reflection'. Gas pushing back on the nose of the Lehigh vent liner is deflected on an angle that will not allow all or most of it to go back through the vent liner. From there you are on your own and you can decide yourself if it of any use to you. Others believe it is.