Weight distribution
He probably means measure your projectiles and sort them based on weight. +/- 0.5gr or whatever.
!
1 grain apart...437 grains being the longest column
So about 0.01% variance
Makes me want to blow the dust of my similar moulds. I made some slugs years ago, and was surprised how easy it was. I have a bunch of real soft lead handy, but sadly have never reloaded shotgun before. I am now tempted, even though my shotgun is not rifled for slugs. Everyone here makes it sounds like a lot of fun.