medalguy said:
Take note of what was said about the reliability of internet information. In 2 of the three posts above this one, there are opinions about 9mm that vary from it's a good safe starting cartridge to it is more forgiving than most loads.
I'm not saying either is wrong, just the opinion of the individual likely based on their experience.
Ah well, there really isn't a simple answer, and we can only make some general statements without getting into a lot of detail. Or we can get into a lot of detail, only to discover we don't have all the information, nor the means to acquire it, which requires good test equipment and methodology. So where does that leave us?
I will go out on a limb and state that I think the 9mm is a very forgiving cartridge. Being of small diameter, it can tolerate greater pressure than most others before achieving catastrophic failure, but being of smaller capacity, there is less total energy available to release. The math to prove that is fairly simple - the math to show the amount of pressure a given charge of powder will generate under varying conditions is not quite so simple, as it tends to be a non linear function.
So more than anything, powder selection complicates matters, as some are not very forgiving in a given application. But as a general rule (and you can look this up in the manuals), the amount of the same type of powder in different cartridges doesn't vary as much as the case capacity does, so more of the fast powders will come closer to filling a 9mm case, which leaves less space for an overcharge. Of course there is still plenty of room for error, since we don't know which powders are likely to build greater pressure with a given % of overcharge. Not to mention how they tolerate giving up their personal space to an intruding bullet. We can assume, and make reasonably good guesses, but most of us don't have access to all data to make truly informed decisions.
Anyway, I was only able to find one load in my manuals for Clays in 9mm that shows a start and a max load. It's a wide range, compared to most others, and that is good. But I don't know anything about the density of Clays, so I don't know if it about fills the case, or hardly fills the case. Or all those other little details that would be nice to know.
One thing I do know (because I checked it), it is possible to double charge a 45acp with a max load of Bullseye, but most 9mm loads won't allow much more than 30%-40% over a max load of Bullseye.