There has been a number of findings from experiments conducted shared with us over the years and, if I remember correctly, the short answer usually boils down to no fancy shapes / bell mouths etc required. Just a nice, parallel, smooth port with no sharp, jagged edges. I think one of the links that T20 posted onto the thread about HW80 transfer port size to the Pyramydairblog shows a few experiments with differing layouts and I'm sure I've seen plenty of others in the past. That old thread is now on page 1, by the way.
. Jim has provided some brilliant information over the years on the subject in Airgun World.
As has been said up above, all very interesting and intriguing stuff.