Again, the Federal 9 MM 124 grain +P+ safe to shoot in modern handguns? I don't mean range shooting, just a magazine or two and the rest for self defense?
"
You pays your money and you takes your choice."
"Safe" is relative, not absolute. It's ALWAYS relative.
The deal with "standards", in the context of this thread, is that manufacturing firearms to a known ammunition standard for a given cartridge results in a firearm which has an "acceptable" safety margin over the service life of the weapon. In layman's terms, this means you are extremely unlikely to have a premature mechanical failure or catastrophic failure of the weapon under normal use.
If you EXCEED the chamber pressures the firearm is built to, then you DECREASE the safety margin. This means you are more likely to have a premature failure of the weapon because you are operating it with higher chamber pressures than it was designed to be routinely operated at.
If a firearm is actually designed for +P ammunition, then obviously using +P ammunition is perfectly acceptable.
If a firearm is NOT designed for +P (and by definition +P+) ammunition, then you ARE operating at a lower margin to safety.
What you are asking in your posting is for an absolute answer to "is it safe". There is no absolute answer, in terms of a line of demarcation in which one can say "cross this pressure and you WILL have a failure/stay below it and you WON'T have a failure".
Let's touch just a tiny bit on a subject called "Fracture Mechanics".
Fracture Mechanics is the study of crack propagation in metals. It is concerned with how metals may fail under a stress in order to characterize resistance to fracture. There are two main modes of failure: ductile (the metal bends) or brittle fracture (the metal cleanly snaps, with no evidence of bending).
The point at which failure occurs, whether ductile or brittle fracture, changes for a given metal/alloy under a variety of conditions as well as over the service life of the metal. Change a metals' temperature, for example, and you change the point at which it will fail at and, potentially, the failure mode.
And the failure point/mode may change over life due to cyclic stress. When you repeatedly apply stresses (thermal and pressure), you may change the metal's fracture toughness (the ability to resist crack propagation) through a variety of reasons, such as the size of the microscopic cracks already present in the metal's crystaline structure or changes to the metal's crystaline structure itself.
Those who are seriously into reloading can tell you that hardening of brass cases through repeated reuse is a real thing and eventually leads to cracks unless they are annealed at some point.
WHAT DOES ALL THIS MEAN?
What it means it the answer to your question is not only a problem with obtaining a precise answer to your question, but that the answer itself may change over the service life of your firearm.
BOTTOM LINE:
Your firearm is not likely to have a catastrophic failure at design +P pressures. But you WILL be adding more stress to your firearm which WILL lead to a premature mechanical failure at some point.
Your firearm is not likely to have a catastrophic failure at +P+ pressures SO LONG AS THOSE PRESSURES REMAIN BELOW PROOF PRESSURES AND ARE NOT OF ROUTINE USE.
But the problem with +P+ is that there is no quantitative definition of what constitutes +P+ pressures. SAAMI does not define any pressure range or limit associated with +P+. They DO, however, define proof pressures.
By convention, it's generally
assumed that +P+ is greater than the defined SAAMI +P pressures, but less than proof pressures. But the rub is "assumed". There's an old saying that when you "assume" something, you make an "*ss" out of "u" and "me". The plain fact of the matter is that there is no defined upper limit and therefore +P+ can not only approach, but exceed proof pressures. This is bad juju. The only way to know what the actual pressures a manufacturer of such ammunition is would be to contact them and ask...and they may or may not provide that information to you.
My personal opinion is that there are a great many choices in quality self-defense ammunition out there which is more than adequate for self-defense. There is very little, if any, benefit to shooting +P or +P+ ammunition.
But as I said above:
"You pays your money, you takes your choice."