Interesting. And here I thought they were all made by hand. I guess shot producers aren't nearly the highly patient and dextrous craftsmen that I made them out to be.
Skill, but not a great deal if it, is definitely involved, and of course patience as you make then two by two.
Shot, on the other hand ... well, my family has been reloading shot shells since the '70s or earlier (my father has casks of Hercules smokeless powder from plus or minus WWII that he's still using), and the regular charge of shot for a 12 gauge shell is 1 oz plus or minus a little. So let's go for the reasonable worst case, dove, which are as about as small a game bird as is practical, and they require a lot more shooting at than the bigger birds because you generally conceal yourself in a location between sources of food and water, so you've leading them in a way which is particularly difficult.
A single 1 oz load would be 337 pellets of #8 shot. It's usual to go through more than one box of 25 per season ... so, oh boy, would that require "highly patient and dextrous craftsmen", and this activity of both sport and procuring tasty animals would be way beyond the reach of the working class.
(Math is fun.)