I often hear that we can never have too many load manuals. If I am producing limited ballistics; specifically 9mm and 9mmM, why do I need more than one load manual plus access to data from the vendors I purchase powder from? Could someone please elaborate?
If we take your question
precisely as it is worded, the argument can be made much like
Brian Pfleuger has done (before as well).
"Need" is different for everyone. The
next guy who posts here who has never touched ANY load manual and has began his entire handloading career with nothing more than YouTube videos under his belt... will not likely be the first such animal.
And there's no telling how he finds his way in this endeavor... perhaps he ends up doing fine. (he'll have to define "fine")
I can say that I own three published and very well known manuals. One of them I refer to multiple times in any given year. One of them I refer to somewhat less because it is outdated. (published in the 1980s) The 3rd manual offers me VERY little because it's much more geared toward rifle cartridges and I am a handgun guy.
With 20+ years at the bench, I could write a dissertation of where -MY- actual
load data has come from over the years and compare it to where I get load data
now. I suppose the two biggest differences between now and years gone by are the internet and my confidence in actually developing my own loads and using published data as a guideline rather than a "recipe."
A reloading manual offers an awful lot more than simply published load data. If what
precisely it offers must be explained -- there's a good chance that if offers precious little to that particular audience.
It's very easy to admit however that a decent published Load Manual tends to be quite a bit less important in the age of the internet than it once was. Of course, some folks may be too jaded to agree, but that's okay too. Those same folks very likely get less out of their internet experience, and may certainly get even more from their Load Manual experience.