Throughout the years I've owned my CS9 I've used whatever duty ammo was available for issue, which has included 147gr, 127gr +P+ and in recent years 124gr +P.
In the armorer classes the usual answer to this question was that we should use whatever good quality factory ammo was approved for use by our agencies, ultimately leaving those choices up to the agencies, but to be aware that using +P (and +P+) loads would accelerate general wear & tear on the guns, and that more frequent replacement of recoil springs was prudent.
I remember in my first class we were told to periodically inspect guns for signs of any unusual wear or conditions, or damage, that might be caused by the increased recoil resulting from higher pressure loads.
One interesting example given was that increased recoil forces could sometimes do things like cause the slide stop assemblies to sometimes acquire either an inward or outward "bend", meaning the rear of the slide stop lever might eventually acquire a greater or lesser angle (than 90 degrees) in relation to the pin of the assembly (the pin that goes in the frame). This was mentioned when we were discussing how some agencies commonly used +P loads.
Back then, armorers were told that a slight bend (increase or decrease) of the lever/pin angle could be adjusted and corrected by judicious and CAREFUL use a soft lead babbitt bar, while the slide stop assembly was carefully held in a vise. In subsequent classes armorers were told to simply replace a slide stop assembly that was found to be out of original spec. The inference seemed to be that it was better to replace a tweaked slide stop with a new one, rather than hoping the armorer was skilled and experienced enough not to cause
more damage by the 'corrective action' than was present in the first place.
Same thing regarding a damaged slide stop plunger and spring. We were originally taught to drift the roll pin out of the slide stop to replace a damaged plunger or spring, and then replace the roll pin. Later on armorer students were taught to simply replace the whole slide stop assembly for a plunger or plunger spring problem.
Granted, if you have the spare assemblies, it's certainly easier and faster to replace the whole thing. Saves time at the bench, and you're using a new assembly that's in fresh factory-spec. It also reduces the chance of an armorer botching the "repair".
Personally, I prefer to restrict my own use of +P or +P+ ammo to those ammo lines offered by the major American ammo companies who make ammo intended for use by LE agencies.
FWIW, a while back I was told by a couple of folks at the factory that the use of +P+ ammo was
not recommended for use in the Shield 9.
Something else to bear in mind is that the designation of +P+ really only tells you that it's higher pressure than +P, but not by how much.
There's a pretty good assortment of choices of standard pressure defensive ammunition being offered nowadays.
Just some thoughts. Can't offer anything definitive. I don't make the guns (or the ammo).
I've usually replaced the recoil springs in my own CS45 & CS9 much sooner than the standard recommendation of every 5 years or every 5,000 rounds offered to armorers. I like to err on the conservative side of trying to minimize wear & tear in those little Chiefs Special pistols, and the recoil springs used in them are pretty short to begin with.
