Originally Posted by
gratewhitehuntr
the issue here is axial compression on soft round shot
that is because having the same volume of shot in a smaller diameter cylinder makes the column longer
the 410 is a rather extreme example of this
it's a very long narrow hull
the 12 is wider than the 20 (think pilsner vs pint mug)\
when the powder ignites the 20 undergoes more axial compression on the shot stack and therefore increase in distorted shot which in turn causes the stream to open up, or pattern to grow in other words
optimally the shot stack will be no taller than it is wide
they call this a "square load'
the 16 is so sweet because of square loads