Bunk,
It is putting powder into a loose stocking and shaking it to sift out the "fines" powdery gunpowder that can cause irregularities in loadings unless it is removed. Then you are left with a consistent grain size of powder and will get more consistent loadings.
Bunk
one country boy to another - you might be able to buy a piece of 40mesh stainless steel screen in an ag supply shop (what we use in sprayer screens) 40 mesh will take out FFFF and finer -- I make me own powder and improvising screens saved me some dollars - just got to carefully measure to make sure ya got the right size - this also works great for anybody that buys 4FA or 5FA powder (fireworks grades) to save some dollars - so I'll write with that in sight.
so ..... in the kitchen shop I found a 12 mesh vegetable strainer, everything thats called FG and 4FA should go through 12 mesh so ya can proly skip that one
..........you will get some aluminium window screen thats 16 mesh - it holds FG and passes FFG
..........back to the kitchen shop again you will find 20 mesh or 24 mesh strainers - so depending on what you shoot most of - 20 or 24 mesh holds FFG passes FFFG. . just make a choice there do ya want more FFG or more FFFG
.......... now your 40 mesh stainless filter screen holds FFFG passes FFFFG (couldnt find that in the kitchen shop but I had some in my workshop anyways - its the important one)
......... I spend a bit of time in rag shops looking for good patch material for my frontloaders and in there you should find a sheer (see through) material that goes right on 100mesh - dunno what its called but the gal behind the counter will enjoy helpin you - imagine watching those old movies - Arabian nights or what? the dance of the seven veils where we all thought this gal is gonna take it all off and didnt - she threw some of those flimsy veils up in the air and a guy slices one of em in two with a big ole sword (scmitar?) - well thats the stuff you want - get a yard of it - makes great filter screen for all sorts of things - I use that to take the mill dust out of my FFFFG -- even just that one if you have dusty powder would work well - better n a sock I reckon
I just make little rectangle screen boxes aboot 10"x 8" outa 3x1 pine and tack/glue/staple the mesh to the bottom - but the kitchen strainers just use em as is. Always go from coarsest screen to finest - thatway any fines you generate in the shaking process end up where ya want em.