Theoretically they allow gas to escape from pierced primers an sometimes along the chamber walls when using brass shells. The hot gas (flame) has been known to chip and sometimes fracture stock heads and to travel back along the plunger channels toward the shooter's face. I have no idea how effective these ports might have been in eliminating or reducing these problems.
This porting process was patented by W. & C. Scott early on.
My apologies to Jim Pasman for so wildly veering from the original topic of this thread.
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