I did check it out. But I'm not sure you read my post correctly:Making your own casings is just really risky. You probably didn't check the link out, but the parts of a case and the techniques required to combine them are very advanced. You do -not- want cracked or failed cases. In another post, I mention how a friend's eye was saved by his safety glasses when the round came apart.
Not for kids, kids.
I imagine the main reason the majority would not make their own casings is because of quality control. But if everyone had machines that could produce identical parts to within a fraction of a percentage of tolerance levels, I don't think that would be such an issue. Right now that sort of machinary is only available on an industrial scale. In 2020 it's available on a much more personal level.
Now, let's talk about rimming.
Very important part of the job. If you don't rim properly you risk the opening being too tight. When that happens, you won't be able to fit in properly. This can be either a waste of time or, much later, a complete fizzle.
The opening has to be wide enough to accept the full circumference, but not so wide the base slips out. Patience and good hands are necessary at this juncture.
Not to worry, though, you can pick up a cheap tool for proper rimming. I've used mine for years with little failure.