I used to do a lot of general chemical powder processing and blending . We had a vibrating classifying sieve in the lab into which you could slot multiple screens in any suitable increment to split a sample into its particle sizes . It would be interesting to take a random sample of say 10 cartridges of brand A , Shell B , and then see what the average distribution of pellet size through something like precise 50 micron increment screens was , +\- nominal . If I was buying shot in from an external supplier I’d sure as hell be doing that on just to see what I was buying .
I can’t imagine anything made as a cheap chuck away , one time, commodity like lead shot can be manufactured to size +\- zero .