I just happen to be looking at a picture in "The Police Shotgun Manual" of a brenneke slug fired thru a diffusion tube. It looks like a mess in the picture (rather deformed) but according to the text:
"The rifled slug's potential is greatly improved with the diverter, as it reduces the angle of yaw. ....With this reduction of yaw and therefore a reduction of air resistance, the accuracy and depth of penetration are improved."
"When fired, the slug is grooved evenly at the top and bottom as the slug passes throught the diverter" (the groove improves the slug aerodynamics)
This article was in reference to a A&W Diverter(tm) made for the AF, it differs from the description of similar devices I've read about (on TFL) in that it has a pair of short vertical ribs extending from the top and bottom of the "duckbill" section...
HTH,
'Crash
(...it's easy to sound smart when you've got the book... )