When loading cast bullets to be shot out of a rifle, where the base/GC hang below the case neck, what realistic velocity can one expect to get from cast bullets before pressure distorts the base of the bullet and accuracy starts going south

Daniel

It depends!

Simple answer to a difficult question ;-)

A rule of thumb says BHN x 1422 = pressure where the alloy starts to "deform". But given the fact that you are squeezing that tail in all directions i doubt you will come in trouble.
The notion that cast bullets should'nt be seated below the neck is to ensure the gas check wont pop off and cause trouble.
And please note that lube might/will kill some of the powder when the tail sticks below the neck.