I thank Britsmoothy for the fine post that shows his digital caliper measuring the size of some of the powder he's using.I have been asked to photograph my powder by someone in this debate.
I have also measured it in metric mm.
There are no warnings or recommendations on the bottle.
View attachment 34992
As others have mentioned, 0.66 mm is equal to .0260 inches. Based on the sizing charts, it turns out that Britsmoothy has not been using his vaulted 4F powder after all.
He has been shooting 3F powder.